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?
so there were 45 reds.
If x were removed, then
(45-x)/(50-x) = 3/4
...