We can make a smaller sample of the problem to help evaluate the big one. For every 19 red, there is 1 black. That means the sample size is 20 in total since 19+1 is 20. In other words, for every 20 marbles, 19 of them are red.
Now we can look at the size of 120. 20 goes into 120 6 times. That means there are 6 times as many red marbles. 19x6=114.
