# Mathematics Aptitude Survey

1. Feb 4, 2009

### kash25

I'm collecting some data for school and need data from as many people as possible. Thanks!

Who can solve this problem in one try without looking at the answer?

A fool wants to tie a rope around the earth. So he buys a rope of 40,000 KM and ties it around the world. His neighbour, also a fool, wants to do the same only he wants the rope on sticks 1 meter above the ground.

How much more rope does he need?

And how much more rope do you need when you use a tennis ball instead of the earth?

Be honest! And be sure to leave a post saying whether or not you get it on the first try!

The answer can be found at http://mindcipher.com/puzzle/42-don-t-hang-yourself [Broken]

Last edited by a moderator: May 4, 2017
2. Feb 4, 2009

### qntty

If the circumference of the earth is 40,000 then the radius is $\frac{20000}{\pi}$. So the circumference in the second instance is $2 \pi \left(\frac{20000}{\pi} + \frac{1}{1000}\right) = 40000 + \frac{\pi}{500}$. Obviously a tennis ball would be the same thing. That was my first try.

3. Feb 5, 2009

He needs an extra $2\pi$ metres of rope, regardless of the radius of the object it was wrapped around.

First try, although I have seen this problem before. (I got it the first try then as well.)

I believe that even if you wrap the rope around a non-circular object (with reasonable restrictions; using a convex object will suffice), you would still have to add $2\pi$ metres to make each point on the rope 1 metre from the object.

4. Feb 5, 2009

### Office_Shredder

Staff Emeritus
We actually had a discussion on this after a math society meeting (I've seen the question before and answered it on my first try for the record). 9 out of 10 university students were able to get the answer right on the first try, the 10th took 10 minutes of explaining because "your notation is stupid"