The school has 800 students with 20 students on the gymnastic team and 10 students on the chess team (including 3 students who are on both teams). how many students in the school are not members of either the gymnastic team or the chess team?

This question seems like its too easy so i might have got it wrong.. but would't you just subtract 20 and 10 from 800 to get 770?

So first you add both gym and chess members then you minus to tht total amount of students: chess+gym=10+20=30   total students= 800 since the question asks how many people did not participate in either gym or chess so u minus: 800- 30 = 770 
so the answer will be 770 did not participate in both gym and chess