(i) Given there are 5 books of mathematics and 6 books of physics. In order to buy one mathematics book number of ways are ⁵C₁ similarly to buy one physics book number of ways are ⁶C₁

Hence, the number of ways a student buy a Mathematics book and a Physics book is ⁵C₁ × ⁶C₁ = 5 × 6 = 30

(ii) Given there is a total of 11 books, so in order to buy either a Mathematics book or a Physics book it means that only one book out of eleven is bought.

Hence, the number of ways in which a student can either buy either a Mathematics book or a Physics book is ¹¹C₁ = 11