Mary takes two marbles, without replacement, from a bag that contains 10 red and 5 white marbles. What is the probability that both marbles are the same color? Express your answer as a common fraction.
The probability that they are both RED = 10/15 x 9/14=90/210 = 3/7.
The probability that they are both WHITE = 5/15 x 4/14 = 20/210 =2/21.
Then, the probability that both are of the same color =3/7 + 2/21 =11/21