Mary is on the student body council. The council will select 3 members at random. There is 50% chance that Mary will be chosen. How many students are there on the council?
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Intuitively, half the council needs to be chosen, so the council must have six members.
For a formal proof, if n is the number on the council, the total number of possible selections is the number of ways to choose 3 from n. The number that include Marla is the number of ways to choose 2 from n-1 (Marla and two others). The ratio of these must be 1/2 from which you can find n.
Hope this helps.
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