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

Ten students of a class are lined up according to their heights. Height of first student is 150 cm, and every next student is 2 cm more in height. What is the total sum of heights of these ten students ?

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.