You may also like

problem icon

14 Divisors

What is the smallest number with exactly 14 divisors?

problem icon

Summing Consecutive Numbers

Many numbers can be expressed as the sum of two or more consecutive integers. For example, 15=7+8 and 10=1+2+3+4. Can you say which numbers can be expressed in this way?

problem icon


Do you know a quick way to check if a number is a multiple of two? How about three, four or six?

Think of Two Numbers

Stage: 3 Challenge Level: Challenge Level:2 Challenge Level:2

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.