3 選 1:
選對 P = 1/3
換之後對 P = 1/3 * 0 = 0
不換之後對 P = 1/3 * 1 = 1/3
選錯 P=2/3
換之後對 P = 2/3 * 1 = 2/3
不換之後對 P = 2/3 * 0 = 0
所以換之後對 P = 0 + 2/3 = 2/3


4 選 1:
選對 P=1/4
換之後對 P = 1/4 * 0 = 0
不換之後對 P = 1/4 * 1 = 1/4
選錯 P=3/4
換之後對 P = 3/4 * 1/2 = 3/8
不換之後對 P = 3/4 * 1/2 = 3/8
所以換之後對 P = 0 + 3/8 = 3/8


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