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:
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
For 3 children (Eldest, Middle, Youngest), each can be B (boy) or G (girl). All possible outcomes:
| # | Eldest | Middle | Youngest | Shorthand |
|---|---|---|---|---|
| 1 | B | B | B | BBB |
| 2 | B | B | G | BBG |
| 3 | B | G | B | BGB |
| 4 | B | G | G | BGG |
| 5 | G | B | B | GBB |
| 6 | G | B | G | GBG |
| 7 | G | G | B | GGB |
| 8 | G | G | G | GGG |
Total possible outcomes
"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
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
Therefore, the probability that the eldest child is a girl given that they have at least one boy is .
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