13. if you are randomly sampling one at a time with replacement
from a bag that contains eight blue marbles, seven red marbles,
and five green marbles, what is the probability of obtaining
a. a blue marble in one draw from the bag?
b. three blue marbles in three draws from the bag?
c. a red, a green, and a blue marble in that order in three draws from the bag?
d. at least two red marbles in three draws from the bag?