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Step-by-step explanation:
there were in total 24 students (asked) in the class.
3 have a brother and a sister (and I assume this means "at least one" in either category).
15 have (at least) a brother but no sister.
4 have no brother but (at least) a sister.
2 have no brothers and no sisters (they are "only children").
a probability is always
desired cases / possible cases.
so, when picking a student randomly, we have 24 possibilities.
out of these 24 there are only 2 that are an "only child".
therefore, the probability to pick an "only child" is
2/24 = 1/12 = 0.083333333...