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Think of a number, double it, then add $3$. Multiply your answer by $4$ and from this take away $5$. Now also take away the number you first thought of.

No matter what your first number was, your answer will be a multiple of which number?

If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.

*This problem is taken from the UKMT Mathematical Challenges.*

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