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'Weekly Problem 2 - 2013' printed from http://nrich.maths.org/

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Your answer will be a multiple of $7$.

Let the number thought of be $x$. Then the final number is $$4(2x + 3) - 5- x = 7x + 7 = 7(x+ 1)$$

This problem is taken from the UKMT Mathematical Challenges.

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