Which term of the series 5, 10, 20, 40, ..... is 1280?

1.	Which term of the series 5, 10, 20, 40, ..... is 1280?
Answer» Clearly, 5 x 2 = 10, 10 x 2 = 20, 20 x 2 = 40,..... So, the series is a G.P. in which a = 5 and r = 2. Let 1280 be the rath term of the series. Then, 5x2n-1 = 1280 2n-1 = 256 = 28 n - 1 = 8 n = 9.

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