↳
So this type of question is to see how you think and if you can do it under pressure. The question is structured to allow you to ask questions, like what kind of plane is it? On the other hand, the interviewer might have an answer from an interviewing book for golf balls in a 757 or whatever he might tell you the plane is. You might have the opportunity to take control, so pick a plane you are familiar with from a recent flight, say “I flew in on a 727 yesterday, so can I use that for my estimate?” (You will probably get a yes answer and he probably does not know a 727 from a 737, so he can’t challenge your numbers if they sound reasonable).
“Ok, when I made reservations on Orbitz and picked a seat, I remember there were 32 seats with 2 on one side of aisle and 3 on the other. I have not done geometry recently, but I think the area of a circle is Pi-R-squared, so a plane is basically a cylinder which means I can multiply the circle area by the length to get volume. (so you can offhandedly ask the interviewer something like “that’s what we learned in junior high right?” – and probably get a confirmation). So it seems like the plane was widest at the floor level and each seat was about 3 feet including arm rests and the space in between, plus the aisle width gets you a diameter of about 18 feet so radius is 9 feet. Let me do a little math over here on the white board: the area of the circle is 3.1415x9x9, so I am estimating and will use 3 instead of PI, so we get 243 square feet.”
“So there were 32 seats and it seemed like about 3 feet for the seat and the tiny leg room x32=96 feet. But there was more space for exit door rows, bulkhead, attendant station, kitchen, bathroom and first class leg space, that adds about 30 feet = 126. The cockpit is probably 12 feet long, but tapers down, so we will use 9, plus 8 effective feet for the tapered tail area = 143 feet. Given the inaccuracy of this estimate, we can ignore the .1415 I dropped off PI to make math easy on the whiteboard earlier. (Back to the whiteboard and) 143 x 243 = 34,749 cubic feet. When I toss the golf balls in, I will assume the seats and equipment is there, but the overhead bins are open for the balls. That other stuff probably reduces usable volume by 20% or 6800 leaving about 28,000 cubic feet. So now I need to figure out the volume of a golf ball.”
“I have only played golf twice, and was really bad – nearly killed somebody with a slice!” (just adding some humor to the interview - so I make a circle with my finger and say) “I think the golf ball was about this big – looks like less than 2 inches, maybe 1 and three quarters? Do you golf a lot? Does that sound reasonable?” (so he might give you a clue or a nod or admit he doesn’t really know either). “Ok, back to the 8th grade. Volume of a sphere – man, that one’s tough, I kind of forget. Area of circle was PI-R-squared, so that has to be multiplied by something like we did for a cylinder. The “height” of a sphere is the diameter, but it is not a full cylinder, so it would be less than multiplying by the length of the “side” or diameter in this case. So, Mrs/Mr interviewer, I know this question is about estimating, not remembering junior high math, so can you give me a hint on this one? (Yes: it’s V = (four/three)*area of the circle) – (or no, just guess. So then you take the volume if it were a cylinder and reduce it by a reasonable amount. He doesn’t know unless he has a book).
“Now we do painstaking math on the whiteboard, ask for a calculator, or take a guess. 3.1414xR2x four thirds = 3.2 cubic inches. Golf balls will leave gaps when packed into a container, but not too inefficient, let’s say instead of 3.2 cubic inches, let’s say 3.4 (just needs to sound reasonable). How many cubic inches in a cubic foot? 1 foot by 1 foot by 1 foot = 12x12x12 inches = 1728. Now divide that by 3.4 and get 508 golf balls in a cubic foot and we estimated 28,000 in the plane, so that is about 14 million golf balls.”
(sidenote, this involves tedious math on paper or whiteboard, but it gets you an answer. It may be that he is happy with you reasoning out your methodology, but he probably has a number in mind, so when you ask, he will let you do it out on the whiteboard)
So I actually did this by scratch just now and it does take a while. I used Excel for the math so it would take longer on a whiteboard. More importantly, not a single one of my numbers is right, but they don’t seem unreasonable. Does the interviewer know how many seats, rows, wasted space, etc on the plane you are using? No. But he sees an analytical thought process and ability to reason and estimate. Don’t panic. Don’t make a random guess right away. I might give a few hints if I asked this question. So if the real answer is 1 million or 50 million then I sounded dumb, but you have to assume he is asking this to everyone else and they might not do much better, so stay confident and calm. Less
