###Human: 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?, ### Assistant: The rectangular tables seat 7*10=70 pupils.
The square tables will seat 90-70=20 pupils.
The library needs 20/4=5 square tables. 
 So the answer is :  5