1) P(3 yellow)
We have 20 marbles in total
The number of sets that can be made is by choosing any 3 of the 20= C(20, 3) = 1140
The set we are interested in is the one where we choose any 3 of the 6 yellow marbles =
C(6,3) = 20
So.....the probability is 20 / 1140 = 2/114 = 1/57 ≈ .0175
2) P ( 1 brown and 2 orange)
We have 5 ways to choose a brown one
And we have only 1 way of selecting both orange marbles
So...the probability is
[ 5 * 1 ] / 1140 = 5/1140 = 1/228 ≈ .0044
3) P( 1 green) = 3/20 = .15