Question

Two balls are drawn in succession out of a box containing 7 red and 3 white balls. Find the probability that the second ball was​ red, given that the first ball​ was: ​(A)  replaced before the second draw. ​(B)  not replaced before the second draw.

Answer 1

a) P(second ball was red | first ball was replaced) = P(red) = 7 / 10 = 0.7

b) P(second ball was red | first ball was not replaced) = P(second ball is red and first ball is white) + P(second ball is red and first ball is red)

                                                                                     = 7/9 * 3/10 + 6/9 * 7/10

                                                                                     = 0.7