How many different selections of 4 books can be made from 10 different books,

(i) Required ways of selecting 4 books from 10 books without any restriction $={ }^{10} C_{4}=\frac{10}{4} \times \frac{9}{3} \times \frac{8}{2} \times 7=210$

(ii) Two particular books are selected from 10 books. So, 2 books need to be selected from 8 books.

Required number of ways if 2 particular books are always selected $={ }^{8} C_{2}=\frac{8}{2} \times \frac{7}{1}=28$

(iii) Two particulars books are never selected from 10 books. So, 4 books need to be selected from 8 books.

Required number of ways if two particular books are never selected $={ }^{8} C_{4}=\frac{8}{4} \times \frac{7}{3} \times \frac{6}{2} \times \frac{5}{1}=70$