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P = .
The denominator = = 2300 is the number of all possible triples of 25 drivers.
It is the total set of events.
The numerator = 4 is the number of favorable triples of the four Smith brothers.
It is the set of all favorable triples.
The ratio P = = = = 0.001739 is the probability under the question. ANSWER
Solved, answered and explained.
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In the other form, this probability is P = = 0.001739, which is the same number/answer.
The last formula may seem to be more self-explanatory to you . . .