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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?

 [Ref: ZWXM] © Kevin Stone

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

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