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.

Find the geometric progression whose 4th term is 54 and 7th term is 1458.

The fourth term of a G.P. is the square of its second term and the first term is -3. Find its 7th term.