In a certain village, it is traditional forthe eldest son (or the older son in a

Question

In a certain village, it is traditional forthe eldest son (or the older son in a two-son family) and his wifeto be responsible for taking care of his parents as they age. Inrecent years, however, the women of this village, not wanting thatresponsibility, have not looked favorably upon marrying an eldestson.

a) If every family in the village has two children, what proportionof all sons are older sons?

b) If every family in the village has three children, whatproportion of all sons are eldest sons?

I know the answers to a) is 3/4
and b) is 7/12

but i am not sure how to come up with those answers In a certain village, it is traditional forthe eldest son (or the older son in a two-son family) and his wifeto be responsible for taking care of his parents as they age. Inrecent years, however, the women of this village, not wanting thatresponsibility, have not looked favorably upon marrying an eldestson.

a) If every family in the village has two children, what proportionof all sons are older sons?

b) If every family in the village has three children, whatproportion of all sons are eldest sons?

I know the answers to a) is 3/4
and b) is 7/12

but i am not sure how to come up with those answers

Explanation / Answer

First list all possible combinations:MF,FM,MM,FF with M=maleand F=female. There are a total of four males. 1 in the MM is eldest,and in the MF and FM, the only son would be eldest. So the answeris 3/4. (1eldest+1only son+1only son) over the total 4. Do the same for 3 children: FFF,FFM,FMM,MMM,MMF,MFF,MFM,FMF There are a total of 12 males with 7 being either the olderone, or just being the only son which makes him the eldestson.

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