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There are 15 people to choose from and 4 places to put them in. How many of the groups include Bob? (This is probability. I made up the problem because I don’t know how to do this and I have a test 4th hour!)

User JayDM
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If we absolutely want Bob in the group, then there are 3 remaining places to fill, and 14 people to choose from. The number of groups is then


\dbinom{14}3=(14!)/(3!(14-3)!)=(14!)/(3!11!)=(14\cdot13\cdot12)/(3!)=364

User Edgarhsanchez
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