There are 5 books on Mathematics and 6 books on Physics in a book shop. In how many ways can a students buy :

 Number of  books on mathematics = 5

 Number of books on physics = 6

 Number of ways of buying a mathematics book = 5

Similarly, number of ways of buying a physics book = 6

(i) By using fundamental principle of multiplication:

Number of ways of buying a mathematics and a physics book $=6 \times 5=30$

(ii) By using the fundamental principle of addition:

Number of ways of buying either a mathematics or a physics book = 6 + 5 = 11