A jar contains 5 black and 2 white balls. If you pick two balls at the same time, what's the probability that one ball is black and one is white?	Choose the correct choice from the following options:
A) 2 / 7
B) 5 / 21
C) 4 / 21
D) 3 / 7
E) 1 / 2

Rationale: p ( 1 st black , 2 nd white ) = 5 / 7 * 2 / 6 = 10 / 42 ; p ( 1 st white , 2 nd black ) = 2 / 7 * 5 / 6 = 10 / 42 . p = 10 / 42 + 10 / 42 = 20 / 84 = 10 / 42 = 5 / 21 answer : b .
Answer: B