Start with a number larger than 0, square it, add 4, double, take away 3, times 4 and finally subtract the original number.

If you were now left with 20, what number did you start with?

Puzzle Copyright © Kevin Stone

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Hint

A little algebra might help.

Answer

1/8.

If we convert the question to algebra we have:

((n^2 + 4) x 2 - 3) x 4 - n = 20

Expanding the brackets and simplifying gives:

(2n^2 + 8 - 3) x 4 - n = 20

(2n^2 + 5) x 4 - n = 20

8n^2 + 20 - n = 20

8n^2 - n = 0 (*)

8n = 1

n = 1/8

Note: in the equation marked (*) zero is also a potential solution, but as the question tells us that we "Start with a number larger than 0" we know that n can't be 0, and therefore we can safely divide by n.