List any 3 numbers. It is always possible to find a subset of
adjacent numbers that add up to a multiple of 3. Can you explain
why and prove it?
How to build your own magic squares.
Weekly Problem 2 -2013
Think of any whole number. Double it and add five. Double this answer and add two. Now take away the number you first thought of. Which whole number will always be a multiple of your final answer?
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.
This problem is taken from the UKMT Mathematical Challenges.