NO.PZ2018062016000082
问题如下:
The stock of AAA company has a 30% probability to rise every year, if every annual trial is independent from each other, the probability that the stock will rise more than 1 time in the next 3 years is:
选项:
A. 0.145
B. 0.216
C. 0.377
解释:
B is correct. Based on the corresponding formula:
p(x)=P(X=x)=(nx)px(1−p)n−x, n = 3 and p = 0.30.
p(2)=(3−2)!2!3!×0.32(1−0.3)1=(3)(0.09)(0.7)=0.189.
p(3)=(3−3)!3!3!×0.33(1−0.3)0=(1)(0.027)(1)=0.027⋅
The required probability is: p(2) + p(3) = 0.189 + 0.027 = 0.216
求解未来三年至少还有一年上涨的概率是否可以想成排出三年来一次都不上涨的概率,即1-0.7*0.7*0.7。但是这种思路算出来的结果与标准答案大相径庭,错误之处在哪里呢?