将四个球随机的放入三个杯子中,以x表示杯子中球的最大个数,求x=2的概率

P(x=2)
= P(2 balls in 1 cup and 3rd ball is in the other cups)
= 4C1.3C2 .(1/4)(1/4).(3/4)
= 4.3.(1/4)(1/4).(3/4)
=9/16答案是8/9，我算出来是2/3P(x=2)= P(2 balls in 1 cup and the other 2 balls in the other cups)= 3C1.4C2. (1/3)^2(2/3)^2= 3. (6) (1/9)(4/9)=8/93C1: choose 1 cup from 3 cups4C2: 2balls in 1 cup and the other 2balls in other cups