You may also like

Consecutive Numbers

An investigation involving adding and subtracting sets of consecutive numbers. Lots to find out, lots to explore.

Calendar Capers

Choose any three by three square of dates on a calendar page...

Days and Dates

Investigate how you can work out what day of the week your birthday will be on next year, and the year after...

Think of Two Numbers

Age 11 to 14
Challenge Level

Here is an alternative and more unusual version of the "Think of a Number" trick which you may have heard of before.

numbers 0-9

Think of two whole numbers under $10$.

Take one of them and add $1$.
Multiply by $5$.
Add $1$ again.
Double your answer.
Subtract $1$.

Add your second number.
Add $2$.
Double again.
Subtract $8$.
Halve this number and tell me your answer.


From your answer I can work out both your numbers very quickly. How?

Choose some different pairs of numbers and repeat the process.
Can you figure out how the trick works?

Click here for a poster of this problem.