Paper moon problem

Algebra Level 2

How many times should I fold a piece of paper, in order to exceed the distance between the earth and the moon?

Paper thickness: 0.07mm

Distance from Earth to the moon: 384,400 km

43 86 17 136

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2 solutions

João Simões
Apr 15, 2021

First fold: 0.00000014

Second fold: 0.00000028

....

Forty second fold: 307,863

Forty third fold: 615,726

So we do on the calculator: Ans = Ans * 2 (starting with 0.00000007 km) counting the number of operations we make a long the way. When Ans reaches >384,400 we stop counting.

Therefore, the number of operations needed to exceed 384,400 is 43 .

Charley Shi
Apr 15, 2021

Each folding of the paper doubles the thickness of the paper. That is, if we fold the paper n n times, the paper's thickness is 2 n 2^n times its original thickness. As a result, 7 × 1 0 5 × 2 n = 3.844 × 1 0 8 7\times10^{-5} \times 2^n = 3.844\times 10^8 2 n = 3.844 × 1 0 8 7 × 1 0 5 = 5.491 × 1 0 12 2^n= \frac{3.844\times 10^8}{7\times10^{-5}} = 5.491 \times 10^{12} n = log 2 ( 5.491 × 1 0 12 ) = 42.32 n = \log_2(5.491 \times 10^{12}) = 42.32 Therefore, the answer is 43 \boxed{43} , because 42 folds still falls short so we need an extra fold.

Thanks for sharing an explanation way better than mine!

João Simões - 1 month, 4 weeks ago

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