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A lvl H2 Maths: Permutations Combinations


a) There is a committee of 5 boys and 3 girls. At a meeting, 2 boys are absent. Find the number of ways they can sit at a round table with 8 seats.

b) A deck of 10 cards is numbered from 0 to 9. Find the number of ways 5 cards can be chosen such that the sum of the chosen cards is larger than those not chosen.


a) Factorial 8 different seats, but 2 are "repeated" as empty seats
Hence, the answer is

(8 - 1)! / 2! = 2520

b) There are 10C5 ways of choosing 5 cards without restriction

The 5 cards chosen is either bigger than or lesser than the cards not chosen.

The probability that the chosen cards are greater than the "unchosen" cards is equal to the probability that the chosen cards are lesser than the "unchosen" cards

Hence we can just take 10C5 divide by 2 = 126

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