a boy has 3 library tickets and 8 books are of his interest. Of these 8,he doesn't want to borrow maths part 2, unless maths part 1 is also borrowed? In how many ways can he choose the 3 books to be borrowed ?
Various cases possible are:
(i) When Maths part-I is borrowed: Here, the boy may borrow maths part-II. So, he has to select 2 books out of the remaining 7 books, which can be done in ways.
(ii) When maths part-I is not borrowed: Here, the boy will not borrow maths part-II. So, he has to select 3 books from the remaining 6 books, which can be done in ways.
∴Total number of ways = +
= 21 + 20