If the third term of an A.P. is 5 and the ratio of its 6th term to the 10th term is 7 : 13, then find the sum of first 20 terms of this A.P.

Given, a 3 = 5 ∴ 5 = a + 2d (By formula a n = a + (n - 1)d ) ⇒ a = 5 - 2d. (Eq 1) Also, Putting value of a from Eq 1 in above equation Hence, the sum of first 20 terms of the A.P. is 550.