A box contains 4 red balls and 4 black balls. One by one, every ball is selected at random without replacement. What is the probability that the fourth ball selected is black?	Choose the correct answer
A) 1/4 B) 1/2 C) 1/2 D) 5/8 E) 2
My complicated version of your simple approach   Let the 4 black balls be BBBBand 4 Red Balls be RRRR   They can be arranged in 8 slots _ _ _ _ _ _ _ _   in (8!)/ (4!x4!)   If the fourth slot is Black ball then the arrangement will be to fill   _ _ _ B _ _ _ _   we have 7 slots and 3 Black (BBB) and 4 Red (RRRR)   They can be arranged in (7!)/ (4!x3!)   Hence required probability = [(8!)/ (4!x4!)]/[(7!)/ (4!x3!)]   =2   2
<result>E</result>
A box contains 4 red balls and 4 black balls. One by one, every ball is selected at random without replacement. What is the probability that the fourth ball selected is black?