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The GMAT Math practice question given below is a sequences and series question on Geometric Progression

If the ratio of the sum of the first 6 terms of a G.P. to the sum of the first 3 terms of the G.P. is 9, what is the common ratio of the G.P?

1. 3
2. 1/3
3. 2
4. 9
5. 1/9

The correct choice is (C) and the correct answer is 2.

The sum of the first n terms of a G.P. is given by , where 'a' is the first term of the G.P., 'r' is the common ratio and 'n' is the number of terms in the G.P.

Therefore, the sum of the first 6 terms of the G.P will be equal to

And sum of the first 3 terms of the G.P. will be equal to

The ratio of the sum of the first 6 terms : sum of first 3 terms = 9 : 1

i.e.

=>

=> r³ + 1 = 9

=> r³ = 8

=> r = 2