Think of two whole numbers under 10.
Take one of them and add 1.
Multiply by 5.
Add 1 again.
Double your answer.
Add your second number.
Halve this number and tell me your answer.
I can work out both your numbers very quickly. How?
Choose some different pairs of numbers and repeat the process.
Can you find a good explanation of how the trick works?
This problem is adapted from the NRICH task Think of Two Numbers with permission of the University of Cambridge. All rights reserved.