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A couple has 3 children each child is equally likely to be a boy or a girl. The probability that the eldest child is a girl given that they have atleast one boy is:

Solution

✅ Correct Option: 2

For 3 children (Eldest, Middle, Youngest), each can be B (boy) or G (girl). All possible outcomes:

#EldestMiddleYoungestShorthand
1BBBBBB
2BBGBBG
3BGBBGB
4BGGBGG
5GBBGBB
6GBGGBG
7GGBGGB
8GGGGGG

Total possible outcomes =8= 8


"At least one boy" means NOT all girls. Remove GGG from the sample space.

Valid outcomes with at least one boy:

BBB, BBG, BGB, BGG, GBB, GBG, GGB

Total valid outcomes =7= 7


From the 7 valid outcomes, those with the eldest child as a girl:

BBB - Eldest is B

BBG - Eldest is B

BGB - Eldest is B

BGG - Eldest is B

GBB - Eldest is G ✓

GBG - Eldest is G ✓

GGB - Eldest is G ✓

Favorable outcomes =3= 3


P(Eldest is girl∣At least one boy)=Favorable outcomesTotal valid outcomesP(\text{Eldest is girl} \mid \text{At least one boy}) = \dfrac{\text{Favorable outcomes}}{\text{Total valid outcomes}}

=37= \dfrac{3}{7}

Therefore, the probability that the eldest child is a girl given that they have at least one boy is 37\dfrac{3}{7}.

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