Naomi is going on holiday and intends to read 4 books during her time away. She selects these books from 5 mystery, 3 crime and 2 romance books. Find the number of ways in which she can make her selection in each of the following cases.
- (i)There are no restrictions.[1]
- (ii)She selects at least 2 mystery books.[3]
- (iii)She selects at least 1 book of each type.[3]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.
- [B1] 10C4 = 210
- [B3] 2 Mystery 2 others = 5C2 × 5C2 =100; B1 for one combination, unsimplifiied
- 3 Mystery 1 other = 5C3 × 5C1 = 50; B1 for second combination, unsimplifiied
- 4 Mystery = 5C4 =5; B1 for third combination, unsimplifiied and total
- Total 155
- [B3] 2M1C1R = 5C2 × 3C1 x 2C1 = 60; B1 for one combination, unsimplifiied
- 1M2C1R = 5C1 × 3C2 x 2C1 = 30; B1 for second combination, unsimplifiied
- 1M1C2R = 5C1 × 3C1 x 2C2 =15; B1 for third combination, unsimplifiied and total
- Total 105
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