Question

in a class of 35, all the pupils play at least 1 game, volleyball, netball and hockey. volleyball only 10, those who play netball only 5, those who play hockey only 3, if 2 play all 3 games how many

Answer 1

so 20 play only one sport, or all three sports.

That tells you nothing about the other 15 who play two sports.
So fill in the missing data, and then actually finish your question, if necessary.