Fun & Games Most liked posts in thread: Puzzel...

I say 20 cm.

My explanation below. Skip past this if you still want to work it out yourself.

1. First, reduce the problem. The string goes around the circumference 4 times on a 12 cm length tube (rod). That's the same as 1 time around a 3 cm length tube, and multiply by 4 for the final answer.
2. So we start with a tube of 4 cm circumference and 3 cm length, and the string goes around it once.
3. Now imagine that we cut the tube lengthwise between where the string begins and ends. Unroll the tube. You will get a rectangle of 3 cm (tube length) x 4 cm (tube circumference)
4. On this rectangle, the path of the string going around the tube 1 time is from one corner to the opposite corner.
5. The string is the hypotenuse of a right triangle with 3 x 4 sides. Applying the Pythagorean theorem, the hypotenuse (string length) is 5.
6. Multiply by 4 to get the final result 20.

 

Bingo^^^^^^^^

Nice. This was actually part of a test I took some years back.

 

Smart kid.

 

I think the puzzle that stumped 96% of the math people was the birthday on mentioned in the article not the pipe. I followed their link to it. Can You Solve the Math Problem That Has Torn Singapore Apart?

 

But for 4 cm diameter, I get 51.678.

4 * sqrt( (4 * pi)**2 + 3**2 )

As the problem said, "Show your work".

 

^Whelp^ You're smarter--and better educated--than I am.

 

The length of the string is the length of the diagonal of the rectangle with

l = 4 \cdot 4 cm = 16 cm\ and\ w = 12 cm

Therefore the length of the string is

s = \sqrt{16^2 + 12^2} = 20\ cm

 

Damn... so that's why the piece from my machinist came back looking kinda funky.