Stage: 2 Challenge Level: 
You could start at either end of the question.
Try making a list and eliminating numbers that do not fit.
Interactivities. Multiplication & division. Prime numbers. Factors and multiples. Trial and improvement. Addition & subtraction. Properties of numbers. Working systematically. Divisibility. Odd and even numbers.
Published December 2000.
Copyright © 2007. University of Cambridge. All rights reserved.
NRICH is part of the family of activities in the Millennium Mathematics Project, which also includes the Plus and Motivate sites.
email: nrich@damtp.cam.ac.uk