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A church hymn book contains 700 hymns, numbered 1 to 700.
Each Sunday the people in the church sing four different hymns.
The numbers of the hymns are displayed to them in a frame by dropping in single-digit boards like this:
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 hymn numbers?
How many of each number must there be?