Jar of Marbles Puzzle

Question

A jar contained 50 marbles. Of the 50 marbles, 90% of them were red and the remainder were green. Some of the red marbles were removed from the jar and after this, 75% of the marbles in the jar were red. How many red marbles were removed from the jar?

The answer I got is apparently not correct

Answer 1

45 of the 50 were red. 90% of 50 is 45

45-x = 75% of 50-x

45-x = (3/4)(50-x) = 75/2 -3x/4

x/4 = 45-75/2 = 15/2

x = 4(15/2) =30 red marbles were removed

which left 20. 15 red/20 total = 15/20 = 3/4 = 75%