A member of book club wishes to purchase two books from selection of ten books recommended for a certain month. In how many ways can she choose them?
A) 47
B) 20
C) 90
D) 45
E) 12

Question

A member of book club wishes to purchase two books from selection of ten books recommended for a certain month. In how many ways can she choose them? ​
A) 47
B) 20
C) 90
D) 45
E) 12

Quizplus · Accepted Answer

The number of ways to choose two books out of ten is equal to the number of two-element subsets of a ten-element set. This can be calculated using combinations, which gives us:
10C2 = (10!)/(2!(10-2)!) = (10*9)/2 = 45

A Member of Book Club Wishes to Purchase Two Books