Question 1170762
There are 30 in the race and 30C3 are the number of ways 3 finishers may occur. That is the denominator.
25C3 are the number of ways to choose 3 seniors from 25
so 25C3/30C3=0.5665
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all juniors is 5C3/30C3=0.00246
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2 seniors and 1 junior is 25C2*5C1/30C3=0.3695
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1 senior and 2 juniors is  is 25C1*5C230C3=250/30C3=0.06158