2014 AMC 8 Problem 18

Attempt Problem 18 of the 2014 AMC 8 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2014 AMC 8 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

18.

Four children were born at City Hospital yesterday. Assume each child is equally likely to be a boy or a girl. Which of the following outcomes is most likely?
all 44 are boys
all 44 are girls
22 are girls and 22 are boys
33 are of one gender and 11 is of the other gender
all of these outcomes are equally likely
Answer: D
Concepts:binomial probabilitycombinations
Small hint:
List outcomes by the number of boys and girls.
Big hint:
The most likely category is the one with the most arrangements.
Solution:
There are 24=162^4=16 equally likely birth-order outcomes. The counts by category are: all boys, 11; all girls, 11; two boys and two girls, (42)=6\binom{4}{2}=6; and three of one gender and one of the other, 2(41)=82\binom{4}{1}=8.
The largest count is 88, so the most likely outcome is three children of one gender and one of the other.
Thus, D is the correct answer.
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