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.

The sum of the first 14 terms of an A.P. is 1505 and its first term is 10. Find its 25th term.

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

If the 4th, 10th and 16th terms of a G.P. are x, y and z respectively, prove that x, y, z are in G.P.