How many different selections of 4 books can be made from 10 different books, if There is no restriction Tow particular books are always selected; Two particular books are never selected?

Explanation

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=\frac8(4 )\times \frac7(3) \times \frac6(2) \times \frac 5(1)=70`