Mark has 5/6 as many blue marbles as red marbles. After buying 20 more marbles of each color, he has 7/8 as many blue marbles than red marbles.How many of each color did Mark had at first?
red: 6x
blue: 5x
(5x+20)/(6x+20) = 7/8
...
blue: 5x
(5x+20)/(6x+20) = 7/8
...