I was confused before when I saw the answer such as: C(4,2)/2^4 = 6/16.
Now I finaly figure out the above approach and would like to share with you:
There are two analysis approaches:
Choice approach:consider the choice of each item(each baby or each dice). each baby have two choices (boy or girl)
So the total choices = 2^4 = 16
So the 2B and 2G choices = First Baby's choice * (Second baby chose the same as first baby + second baby chose the different from first baby * the third baby's choice) = C(2,1) * (C(1,1) + C(1,1) * C (2,1)) = 2 * (1+2)=6
Result approch: consider the final result as whole. (how many are boyes and how many are girles)
So the total results = zero girl + 1 girl + 2 girls + 3 girls + 4 girls = C(4,0) + C(4,1) + C(4,2) + C(4,3) + C(4,4) = 1 + 4 + 6 +4 +1 =16
So the 2B and 2G results= C(4,2)= 6
It is easy to get the total number from the choice approach and is easy to get the spec number from result approach. So once you fully understand the issue, you can use the mixed approach like those "Da Niu" C(4,2)/2^4
Hope I am not making you more confused.作者: may 时间: 2002-7-22 22:45
