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Think of any whole number.
Double it and add five.
Double this answer and add two.
Now take away the number you first thought of.
Your answer will always be a multiple of which number?
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.
An investigation involving adding and subtracting sets of consecutive numbers. Lots to find out, lots to explore.
Choose any three by three square of dates on a calendar page...
Investigate how you can work out what day of the week your birthday will be on next year, and the year after...