A gardener is going to plant 2 red rosebushes and 2 white rosebushes. If the gardener is to select each of the bushes at random, one at a time, and plant them in a row, what is the probability that the 2 rosebushes in the middle of the row will be the red rosebushes?
A. 1/12
B. 1/6
C. 1/5
D. 1/3
E. 1/2
1.自己排一下吧,红的R,白的W,可能的排法也就6种RRWW RWRW RWWR WRRW WRWR WWRR;
2.或者你可以看作先把2朵白的或者2朵红的种下去,这样就留下3个位子给没种下去的2朵,计算还没种下去的2朵的种法,C21*C31(位子可以重叠,重叠的话就是两朵在一起);
先做全排列P(4,4),考虑到白与白相同,红红相同,最终排列可能数,P(4,4)/(P(2,2)P(2,2))=6, 所以结果1/6.
两个红的在中间固定后只有1种方案
全部排列有:C(2,4)
4个洞,取2个种红的,剩余种白的
所以是1/6
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