Zoran Interview Question: You have 2 pieces of rope, ea... | Glassdoor

Interview Question

ASIC Verification Engineer Interview Sunnyvale, CA

You have 2 pieces of rope, each of which burns from one end

  to the other in 30 minutes (no matter which end is lit). If different pieces touch, the flame will transfer from one to the other. You cannot assume any rope properties that were not stated. Given only 1 match, can you time 45 minutes?
brain teaser

Interview Answer

49 Answers


Take one rope (Rope A), place it down as a circle. Light match and start burning rope A at the tips that are touching. When the rope completely burns out, 15 minutes will have passed (since both ends are burning and being consumed at once). Hold the second rope (Rope B) straight and place one end so that it will immediately catch fire when the two burning points from (Rope A) finally touch and are just about to burn out.

Thus 15 minutes on Rope A + 30 minutes on Rope B gives you 45 mins.

S.T. on Sep 10, 2010

How about this: Fold the first rope double so the ends touch. Lay it down and lay the second rope so it touches the fold of the first rope. Light the ends of the first rope. After 15 minutes the second rope should ignite. Once second rope finishes burning it is 45 minutes. Same principle as above, I just don't want to sit there for 15 minutes in order to light the second rope.... :-)

F on Sep 11, 2010

Make a T. Simple

Anonymous on Sep 11, 2010

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** You cannot assume any rope properties that were not stated
Burn like this *-------- ===>
After 30mins, Rope A finished burning, and both ends of Rope B start burning

=) on Sep 11, 2010

burn one rope, wait till it gets to the half way point, then you transfer the first one to the second one to initiate the other flame. wait till the end. 45 minutes are up

the guy. on Sep 12, 2010

You have 2 pieces of rope, each of which burns from one end to the other in 30 minutes (no matter which end is lit). If different pieces touch, the flame will transfer from one to the other at the point at which the burn rate consumes the first rope to its point of contact with the second rope. The only thing we know about the second rope is that it will burn in 30 minutes if ignited from one end. There is no assurance the second rope will ignite, or burn at the stated rate, if an end is not made to point of contact.

eaasy on Sep 13, 2010

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depends on what the ropes are made of

johnsoldwoman on Sep 13, 2010

@eaasy you can assume that the second rope with burn at the same rate if lit from the middle, as the rope burns a set amount of time from either end. If Point A to Point B is the same as Point B to Point A, igniting from the center would cause the flame to finishing burning Point A AND point B at the same time.

As for ignition, you have a point. There is no assurance that the first rope does not have a prolonged burn rate at the ends and an accelerated burn rate in the center. Therefore the second rope could have ignited before the first rope finishing burning (at any point before the 30 minute limit) which would make timing 45 minutes improbable.

Anonymous on Sep 13, 2010

Lay one down, and have the second one touch the first rope perpendicularly at the midpoint. Light the first one, when it gets to the midpoint (15 minutes), the second will start burning. When the second one extinguishing you have 45 minutes.

Bguy on Sep 14, 2010

T. Simple, quick, walk away and do 45 minutes' work where you can still see light from the flame. Both ends will finish burning at the same time if environmental conditions are consistent.

awkrr on Sep 15, 2010


TB on Sep 15, 2010

lay ropes end to end, to make one long length, when first length ignites the end of the second length, touch the last remaining end to it and let the flames meet somewhere in the middle. That way each flame will travel in the correct direction, are considered to be burning end to end, without ambiguously being lighted in the middle and hoping the direction will be as desired.... since it takes 30 minutes end to end, then to ignite both ends of the second rope will draw a 15 minute interval no matter what the burn rate is per divided section, thank you and I will accept $150 K per year and 30 days vacation time yearly as well... I had no idea that rope burning was so lucrative a career field... and I want my own rest room.

pat on Sep 15, 2010

it all depends on what kind of match you have. They said you have one match (no book or box) so if it's not a strike anywhere match then you're screwed. However, if it is then make the T formation... The vertical rope burns 30m then the horizontal burns both directions for 15. No loops, circles or folding required :-)

Josh on Sep 17, 2010

burn A at the two ends will give you 15 min, and when finish A, then burn B next.

jake on Sep 17, 2010

Lay the map in a T formation, light two ends of the horizontal rope with the match. when the flames meet in the middle in 15 minutes they will light the second rope which is perpendicular to (and touching) the first rope. That rope will take 30 minutes to burn. Hence, 15 plus 30 = 45 minutes.

Lyle on Sep 18, 2010

burn A at the two ends will give you 15 min, and when finish A, then burn B next

Anonymous on Sep 19, 2010

burn one rope, wait till it gets to the half way point, then you transfer the first one to the second one to initiate the other flame. wait till the end. 45 minutes are up
  Helpful Answer? Yes | No

Anonymous on Sep 19, 2010

Lay the two ropes in a "T" shape. Burn the one rope at the bottom end of the "T". It will take 30 mins for the flame to go through first rope, and when it reach the other end, transfering the flame onto the second rope right at the middle with the flame move towards both directions, taking 15 mins to completely burn out.

Ron Asuncion on Sep 19, 2010

This is a silly riddle.... because we're not given the property that the rope can bend, the best formation for the two ropes is to lay them in the "+" formation that everyone got. However, no one mentioned burning the rope at the intersection of the two ropes, ropes will burn in 15 minutes. Also, there isn't anything saying you can't light the ropes more than once with one match (assuming you have the ability to light the match, as someone already pointed out as a significant problem), in that case, you could burn the ropes up in less than a minute. If the rope can bend, just bundle the ropes up into a ball and light the rope ball on fire... it take 1.3 seconds to burn the ball if you create the most efficient weave... true story, I was there. But of course, if you wanted to make the most efficient use of your time (e.g., if you're lazy), you can just crumple the ropes up together, drop the heap of rope on the ground, and throw the lit match on the pile and be done with riddle (and the ropes... and the match).

Turck on Sep 21, 2010

I agree with a couple people here. In this you cant assume the rope burns
Therefore loop one rope and put the end of the other so it touches where the ends meet.

so you have this C--- light the straight rope, when it burns it'll light the "C" which will burn in half the stated time (15 mins).

Dan on Sep 22, 2010

Just a quick one for all the "T" people, you're wrong... The rope doesn't burn uniformly so you can't measure to choose time. If you could you may as well just cut one of the ropes...

Dan on Sep 22, 2010


Most likely solution to work: Straight followed by loop.


One straight, followed by a loop. Ignite straight first, have it ignite the loop.
You ignite the straight, gives you 30 minutes.
Loop ignites, both ends may burn at different speeds, but when it finishes it will be 30mn/2. The flame may end anywhere on the loop, not necessarily at the 1/2 way of the rope.

Why wouldn't it work:

Above, you're assuming the rope burns end to end.
If the rope is really non-uniformly burning, you can devise a case where burning on both sides at the same time doesn't make the burn 1/2 the time:

For example, the outside burns real fast and the core real slow: no matter where you ignite it, or how many times, it would burn in 30mn.
For that case, you could split the rope at their diameter and maybe rig a solution. But likely there can be another counter example that can be built, more and more far fetched.

Interviewer will want you to explain the thoughts, non-uniform burn, etc. So this is an opening to check your deductive powers.

ILikeNetApp on Sep 22, 2010

What's the purpose of such a simple question? To judge how you react and explain yourself?

Really on Sep 23, 2010

Fold each rope in half & mark them in the middle. Light 1 rope & when it reaches the middle light the other one. When the 2nd rope burns completely you'll be at 45 minutes.

Gotit... on Sep 23, 2010

no comment

Anonymous on Sep 25, 2010

Too easy... fold one of the ropes in half end to end with the full rope... viola!

Bill on Sep 25, 2010

There are a couple of answers. It depends how you would interpret the question.

"the flame will transfer from one to the other"
Assuming no special materials you would have on the ropes. This phrase can be interpreted that THERE CAN ONLY BE ONE FLAME. Thus the flame moves from one to the other and does not multiply i.e you can have more than one rope or section of the rope burning .(If it could the solution is easy-just arrange both ropes end to end into a pentagon shape or do what some people have suggested above). for the 45 mins thing you do the T shape like people above suggested so 15+30 =45. If you assume you HAVE to take 45 mins this is the best way. Assume that an earlier finish is best, the Pentagon would be fast. -When I mean pentagon I mean the 5 pointed star thing you know from the occult.-

If there was only one flame which could transfer from one rope to the other without creating multiple flames you would have to ask how fast does the flame transfer and what distance does the flame burn at before being transferred. The reason for this is, if the ropes are lied down next to each other TOUCHING each other then the flame would burn, transfer, burn, transfer etc. this would take 30 mins assuming that there is no time lapse on the transfer(hence my questions above). If you wanted to hit the 45mins like it said in the question just have both ropes lie next to each other but have the second rope start from the middle of the first. So after 15mins the second will burn and the flame from this point the remainder of the first rope(15mins) and the second rope will burn 30 mins.

Sorry for the poor spelling .


stoneyzatiger on Sep 26, 2010

Take one rope, fold it over so one end touches the other, then cut the rope in half. Place one half of the rope at the end of the second whole rope so their ends are touching. Then light either end for 45 minutes of burn.

I can't assume rope properties, but nothing says I can't cut it in half.

tsplano on Sep 26, 2010

Fold Rope A in half. Lay it down next to Rope B, which is not folded at all. Light Rope B.

Shannon on Sep 27, 2010

Reminds me of some of the stupid questions in the Corvirtus test. I took one recently and felt like a 3rd grader. I'm Schmart, I swer.

Steve on Sep 27, 2010

Make a plus sign the the ropes. Light any one end. 15 mins to get get to center, 30 to burn the other rope.

Shane on Sep 27, 2010

Must be a crew of techies answering this question. The obvious answer is to look at a clock.

Craig on Sep 29, 2010

lay both pieces of rope down so that end of rope A touches the end rope rope B light the end of either rope, after 30min have passed the flame will transfer to the other rope,when the rope is half burn 45 total min will have passed. so yes you can time 45min.

matt on Sep 29, 2010

45 minutes is easy. I would rather have an hour of light. Tie the ropes together, making one long rope. Light one end and get 1 hour.

Bill on Sep 29, 2010

The Question is "Given only 1 match, can you time 45 minutes?". Option 1. You have a timing device and are allowed to use it - no prohibition stated in setup or question - of course you can "time 45 min" to the precision of your timing device. Option 2. We know: Ignition can occur at either end. From reaction start to reaction end of each rope takes 30min. We can not assume that the rope burns evenly, can be bent, lit up other than the ends or even transmit the flame to the outside of the rope (Think trapped chemical burn in a tube). Therefore we can NOT build a timing device.

Me on Sep 30, 2010

Put one rope in a straight line, bend the second rope into an O shape (with both ends touching one end of the first rope), when the first rope reaches its end, 30 minutes will have passed, and ignite the second rope. The second rope, which has both of its ends ignited at the same time, will meet at its center, timing 15 more minutes have passed.

-> ///////////O

Steven on Oct 5, 2010

Lay down both ropes evenly side by side. Slide one rope down 1/2 the length of the other then light one of the ends.

Bruce on Oct 6, 2010

Make a "T" with the ropes. 1 rope burns in 30 min, that rope is touching the middle of the second rope, and with two equal halfs(30/2) it should burn in 15 mins.

Really.... on Oct 6, 2010

      -------------, then push them together

nope, like this on Oct 6, 2010

First of all, the question is "Given 1 match, can you time 45 mins?"... well you can stand there with a match in your hand and time 45 mins with any stopwatch. Or is the question really can you time 45 mins using that match? Then using the match to create a shadow..... hem hem maybe not.
Let assume the question was meant to be "can you burn both ropes in 45 mins?"... well it doesn't say anywhere that the match can only light one rope. I've lit many birthday cakes in my days and I promise that only one match can light a full array of candles. So if the ropes burn in 30 mins. and you light both of them with the one match then yes; the burning of the ropes should be done under 45 minutes. But then again.... maybe the question was "can you burn both ropes in EXACTLY 45 minutes?"
Now the real answer I think is that the question need to be more clear :)

Why make it hard on Oct 8, 2010

Light one of the ropes from one side only. when it burns out completely (30 mins) light the second rope from BOTH sides. that will double the rate at which it burns, making it completely burnt out in 15 mins.
Total 45 min

islamomt on Nov 22, 2010

didn't realize the "one match only"
make a loop and a long rope running out of it (like a key figure)
light the loose end of the standing rope. it will burn in 30 min. When it reaches the end it will ignite both sides of the other rope, doubling the rate at which it burns (i.e. it will burn to completion in 15 min only) that's a total of 45 min.

islamomt on Nov 22, 2010

Keep the first rope straight, and the two tips of second rope touching one end of first rope. Now lit the other end first rope. So first rope takes 30 min to burn and transmit fire to two ends of second rope which will take 15 min to burn. So total of 30+15= 45 min

Leela Naga sasidhar on Apr 19, 2013

The answer is simple... Make a sundial with the match and the rope.

Kill4Greed on May 17, 2014


Anonymous on Jun 22, 2015

Keep the ropes in this fashion----------》
And fire dem with opposite side burning. Now when both fire meet it wil be 15 mints...nw both have 15 mints remaining now bur first remain rope which will take 15 mints more and den burn second remaining rope which will take another 15 mints..so total 45 mints..isn't it simple ..;)

Anonymous on Jul 8, 2015

The question does not state that the items in the question are required to provide the answer so...

Take note of your start time.

Take the first length of rope and lay it next to the match.

Then, carefully inspect the second length of rope for any defects.

If none are found, or even if there are defects present, place the rope on the ground in a straight line facing north to south.

Place the first length of rope, which is tired to the match, approximately 5 feet to the north of the second length of rope.

**Warning: This step requires quick action on your part**

While the match is burning, tie the first length of rope to the match.

Take the second length of rope and touch the north end to the match while it is still lit.

Grasp the second length of rope in your left hand and spin 360 degrees counter clockwise and throw it as far as you can.

Now go have a beer and keep an eye on your watch to see when the 45 minute timeframe has finished.

Sgt Sugarcoat on Feb 10, 2016

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