Sum
In how many ways can a boy invite his 5 friends to a party so that at least three join the party?
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Solution
The number of ways of inviting at least three friends from 5 friends
= `""^5"C"_3 + ""^5"C"_4 + ""^5"C"_5`
= `(5!)/((5 - 3)!3!) + (5!)/((5 - 4)!4!) + 1`
= `(5 xx 4)/(2xx 1) + 5 + 1`
= 10 + 5 + 1
= 16
Concept: Properties of Combinations
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