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Song Book

Stage: 2 Challenge Level: Challenge Level:3 Challenge Level:3 Challenge Level:3

A school song book contains 700 songs, numbered 1 to 700.

Each month at a special assembly, the children sing four different songs.

The numbers of the songs are displayed to the children in a frame by dropping in single-digit boards like this:

number 2

The board for 6 may be turned upside down to serve as a 9.

What is the minimum number of small boards that is needed to show any possible combination of four song numbers?

How many of each digit must there be?