PS

joywzy

12. In a certain animal population, for each of the first 3 months of life, the probability that an animal will die during that month is 1/10 . For a group of 200 newborn members of the population, approximately how many would be expected to survive the first 3 months of life?
   (A) 140
   (B) 146
   (C) 152
   (D) 162
   (E) 170
ans:B

the survival rate=9/10, X/200=9/10 => X=180

我算了几遍,都是这个结果,我错在哪里呢?

猫猫头

200*0.9^3

joywzy

以下是引用猫猫头在2003-3-7 16:37:03的发言：
200*0.9^3

为什么?

猫猫头

???
200*(1-0.1)survive in the first month
200*(1-0.1)(1-0.1)=survive in the second month
200*(1-0.1)(1-0.1)(1-0.1)=survive in the third month
clear now???

xmwangyu

NOTICE "each of the first 3 months "

joywzy

谢谢猫猫和xmwangyu！！明白了。

猫猫头

不用谢^_^