Question

a jar contains 5 times as many yellow marbles as red marbles, when 12 yellow marbles are removed, there are twice as many yellow marbles red marbles in the jar. How many yellow marbles were originally in the jar?

Answer 1

Let r represent the number of red marbles in the jar. The problem statement tells us ...
  5r -12 = 2r
  3r = 12 . . . . . add 12-2r
  r = 12/3 = 4

The number of yellow marbles originally in the jar is 5*4 = 20.