You may also like

problem icon

Lesser Digits

How many positive integers less than or equal to 4000 can be written down without using the digits 7, 8 or 9?

problem icon

Mini-max

Consider all two digit numbers (10, 11, . . . ,99). In writing down all these numbers, which digits occur least often, and which occur most often ? What about three digit numbers, four digit numbers and so on?

problem icon

The Codabar Check

This article explains how credit card numbers are defined and the check digit serves to verify their accuracy.

The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice.
Copyright © 1997 - 2019. University of Cambridge. All rights reserved.
NRICH is part of the family of activities in the Millennium Mathematics Project.