How many different selections of 4 books can be made from 10 different books, if
two particular books are never selected.
It is told that two books are never selected.
It is similar to selecting 4 books out of remaining 8books as 2 books are already removed.
Let us assume the choosing the no. of ways be N2,
⇒ N2 = choosing 4 books out of 8 books
⇒ N2 = 8C4
We know that 
And also n! = (n)(n – 1)(n – 2)…………2.1
⇒ 
⇒ 
⇒ 
⇒ N2 = 70 ways
∴ The required no. of ways are 210, 28, 70.
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