SAT概率题求解答
Five red balls and two blue balls are placed in an empty box.One ball a time is to be selected randomly and removed 4 balls from the box.What is the probability that the first two balls removed will be red and third ball removed will be blue?
A 4/21 B 2/21 C 5/8 D 5/7 E 3/42
最好能用组合或者排列的那个Pmn Cmn的公式做，