Given:
The sum of two terms of GP is 6 and that of first four terms is 
To find:
The sum of first six terms.
Solution:
We have,


Sum of first n terms of a GP is
...(i)
Putting n=2, we get


...(ii)
Putting n=4, we get



(Using (ii))
Divide both sides by 6.
Taking square root on both sides, we get

Case 1: If r is positive, then using (ii) we get
The sum of first 6 terms is




Case 2: If r is negative, then using (ii) we get
The sum of first 6 terms is




Therefore, the sum of the first six terms is 7.875.
Subtract original amount from new amount:
84 - 39 = 45
He bought 45 books.
Answer:
B. 74 / 100
Explanation:
I got it right on the test :)