The first thing to do here is to calculate the probability of selecting a red or a green marble
Total number of marbles = 7 + 3 + 2 + 8 = 20
Probability of selecting a red marble is 7/20
Probability of selecting a green marble is 3/20
The probability of selecting a red or a green marble = Probability of selecting a red marble + Probability of selecting a green marble = 7/20 + 3/20 = 10/20 = 1/2
Now our selection spans 250 times, the number of times it should have been a green or a red marble = The probability of selecting a green or a red marble * number of selection times = 1/2 * 250 = 125 times
