Answer: Choice A) 3/40
Check out the diagram below. I have labeled the marbles 1 through 20 to help count out them easily.
We have 3 blue marbles out of 20 total. The probability of selecting blue is 3/20.
If we put the first selection back, then we will still have 20 marbles overall. The are 10 green marbles out of 20 so the probability of getting green is 10/20 = 1/2
Multiply the probabilities 3/20 and 1/2 to get (3/20)*(1/2) = (3*1)/(20*2) = 3/40
