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Let a, b and c be in G.P. with common ratio r, where a ≠ 0 and 0 < r ≤ 2 1 . If 3a, 7b and 15c are the first three terms of an A.P., then the 4 th term of this A.P. is :

1. a
2. a 3 7
3. a 3 2
4. 5a

Correct answer: a

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