Question 810229
You have 7 red marbles and 5 black marbles for a total number of 12 marbles, right? When you add 60, you have a new total of 72, correct? To keep the same ratio of red/total, we have the proportion
(1) 7/12 = red/72 or
(2) red = 72*7/12 or
(3) red = 42
That is, you will have 42 total red marble and 
(4) black = 72 - 42 = 30 or we have a total of 30 black marbles
The ratio of red to black is
(6) 42/30 = 7/5, the same as before we added 60 marbles.
Answer: We should add 35 red marbles and 25 black marbles (total of 60 marbles).