ok now, the total number of games he's played is 500 + x.
the number of games he's won is 370 + x
The number of games he has won is 75% of (500 + x). (cause it says that he won 75% of the total games played)
hence, 370 + x = 75/100 * (500 + x)
370 + x = 375 + 0.75 x
0.25x = 5
x = 20