Question 1000580
a)  There are 190 ways to choose 2 marbles, that is 20 marbles taken 2 at a time is 20! / (2! * (20 - 2)!) = (20 * 19) / 2 = 190,
there are 5 * 4 possible sets that include 1 green and 1 red, therefore
Probability(1 green, 1 red) = ((5*4) / 190) = (20 / 190) = 2 / 19