Find the sum of first 20 terms of an A.P. whose nth term is 15 - 4n.

Solve the equation 2 + 5 + 8 + …. + x = 155.

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.