How many terms of the series 2 + 6 + 18 + …… must be taken to make the sum equal to 728?

In a G.P., the ratio between the sum of first three terms and that of the first six terms is 125 : 152. Find its common ratio.

If the sum of 1 + 2 + 2² + …….. + 2^{n - 1} is 255, find the value of n.