NRICH: Think of Two Numbers
Think of two whole numbers under 10, and follow the steps. I can work out both your numbers very quickly. How?
Here is an alternative and more unusual version of the “Think of a Number” trick which you may have heard of before.
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?
Age 11 to 14
Click here for a poster of this problem.
What are you looking for?
Organization
Website URL
Type of Resource
PDF File
Assigned Categories
Resource k12
Think of two whole numbers under 10, and follow the steps. I can work out both your numbers very quickly. How?
Here is an alternative and more unusual version of the “Think of a Number” trick which you may have heard of before.
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?
Age 11 to 14
Click here for a poster of this problem.
What are you looking for?
Organization
Website URL
Type of Resource
PDF File