Asurvey of 500 television watchers

produced the information:
285 watch football games
195 watch hockey games
115 watch basketball games
45 watch football and basketball
70 watch football and hockey
50 watch hockey and basketball
50 do not watch any of the three games

a. how many people in the survey watch all the three games?

b. how many people watch exactly one of the three games?

Let the number that watch the three games be x, then
50 - x watch hockey and basketball only
70 - x watch football and hockey only
45 - x watch football and basketball only
115 - (45 - x) - (50 - x) - x = 20 + x watch basketball only
195 - (70 - x) - (50 - x) - x = 75 + x watch hockey only
285 - (45 - x) - (70 - x) - x = 170 + x watch football only
50 do not watch any of the three games.

x + 50 - x + 70 - x + 45 - x + 20 + x + 75 + x + 170 + x + 50 = 500
480 + x = 500
x = 500 - 480 = 20

a.) therefore, 20 people watch all the three games.

b.) 20 + x + 75 + x + 170 + x = 265 + 3x = 265 + 3(20) = 265 + 60 = 325.
Therefore, 325 people watch exaclty one of the three games.

I’m not sure what it is i sorry : ((