This is a nice development of the classic “Think of a Number” problems that are a good way of introducing algebraic equations. Nrich covers this introductory activity with Your Number Is… and other linked activities.

This one takes it a stage further. Proving algebraically why this works would be a nice extension activity. Note that with the numbers below, this will only work in 2015. I’ll leave you to work out what you need to change for subsequent years!

*Work this out as you read.*

*Be sure you don’t read to the bottom until you’ve worked it out.*

*First, pick a number 1 to 7*

*Multiply this number by 2.*

*Add 5.*

*Multiply it by 50. (Come on, you can do that without a calculator).*

*If you have already had your birthday this year, add 1765*

*If you haven’t, add 1764*

*Now, subtract the four digit year that you were born. e.g. 1984*

*You should have a 3 digit number:*

*The first digit of this was your original number.*

*The next two numbers are your age.*

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