8,16一道几经请教

wretdgdh

有一题是说40个不同整数，偶数的个数为多少：1)取出一个数，此数为偶数的概率是x/y（数字忘了）；2)取出两个数，均为偶数的概率是2/13。我选了a，2)是怎么也算不出整数解来的。

petqueen的816几经, 偶也做不出来. 可是这不像ets的思路阿,要不就可以确定,要不就不可以确定,则么在整数小数上搞??

himba

check this out:

let x denote the number of even numbers in set N(where set N contains 40 integers), then the Pr(picking two numbers and both will be even) = C(x, 2) / C(40, 2) = x! / (2!(x-2)!) * 40! / (2!38!) = x(x-1) / 40*39 = 2/13 => x^2 - x = 240 => x^2 - x - 240 = 0 => (x-16)(x+15) = 0=> x = 16

confirm: c(16, 2)/c(40, 2) = 2/13