Answer: There are 114 red marbles contained in the bag containing 120 marbles.
Explanation:
To find the amount of red marbles in the bag:
From the question, we can presume that for every 19 marbles in a bag, there exist a single black marble. This is the same case for the total number of marbles (both red and black) in each set is 20 marbles
Set of marbles: 120/20
Red marbles:
19r = 19 x 6
19r = 114 red marbles.
I hope this helps!