A library has 7 rectangular tables and the remaining tables will be square tables. A rectangular table seats 10 pupils while a square table seats 4 pupils. How many square tables are needed so that 90 pupils can read at the same time?
The rectangular tables seat 7*10=<<7*10=70>>70 pupils.
The square tables will seat 90-70=<<90-70=20>>20 pupils.
The library needs 20/4=<<20/4=5>>5 square tables.
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