Copyright © University of Cambridge. All rights reserved.
'Double with 1-9' printed from http://nrich.maths.org/
Using each of the number cards $1$ to $9$ once and once only, find two whole numbers one of which is double the other.
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.