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 by putting together single-digit cards like this:
The card for 6 may be turned upside down to serve as a 9.
What is the minimum number of digit cards that is needed to show any possible combination of four song numbers?
How many of each digit must there be?