There are 12 marbles in a bag, and the marbles are either yellow or green. Two marbles will be randomly picked from the bag, without replacing the first one picked. The probability that both marbles will be yellow is

Question

5
33
. How many YELLOW marbles are in the bag?

Answer 1

x yellow
12 - x green

first draw
x/12
second draw
(x-1)/11
so both
(x^2-x)/132 = 5/33

33(x^2-x) = 660
x^2 - x - 20 = 0
(x-5)(x+4) = 0

x = 5 because it is hard to visualize a negative yellow marble