### Lesser Digits

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

### The Codabar Check

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

### Six Times Five

How many six digit numbers are there which DO NOT contain a 5?

# Mini-max

##### Age 11 to 14 Challenge Level:

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?