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Interview Question for ASIC Verification Engineer at Zoran:
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?
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The answer should be like this
Rope A: --------
Rope B (folded): ===>
Burn like this *-------- ===>
After 30mins, Rope A finished burning, and both ends of Rope B start burning
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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.
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uniformly.
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).
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Most likely solution to work: Straight followed by loop.
---O
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.
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"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 .
Cheers
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I can't assume rope properties, but nothing says I can't cut it in half.
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-> ///////////O
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-------------, then push them together
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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 :)
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Total 45 min
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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.
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7 of 11 people found this helpful
by S.T.:
Thus 15 minutes on Rope A + 30 minutes on Rope B gives you 45 mins.