Let's assume there are 100 games :

.75 * 100 * .35 = games won so far

.25 (100) x games remaining times the percentage needed to get to .5(100) <==== wins = losses

.75 (100) .35 + .25 (100) (x) = .5 (100)

26.25 + 25 x = 50

25 x = 23.75

x = .95 You need to win 95% of the remaining games