I recently asked my friend Alex to buy me some ribbon for my child's bonnet.

Alex went to the haberdashery shop for the required length, but accidentally swapped the feet and inches.

When I measured the resulting ribbon, I only had 5÷8 of the length I required.

How much ribbon did I originally ask for?

Remember that there are 12 inches in a foot.

Puzzle Copyright © Kevin Stone

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Hint

A little bit of algebra might help.

Answer

I asked for 7 feet, 4 inches and Alex brought me 4 feet, 7 inches.

Reasoning

If we label what I asked for as A feet and B inches, which is (12 x A + B) inches, then I actually received B feet and A inches, which is (12 x B + A) inches. Which means that:

(5 ÷ 8) x (12 x A + B) = (12 x B + A)

Simplifying gives:

A = (91 ÷ 52) x B

As B is the number of inches, it can only be between 1 and 12 and must give A as a whole number of feet.

B = 4 is the only possible value, which means that A = 7.

So I originally asked for 7 feet, 4 inches.

Double-Checking

I received 4 feet, 7 inches = 55 inches.

I asked for 7 feet, 4 inches = 88 inches, and 88 x (5 ÷ 8) = 55.