Sequences and Series

Let $$a$$, b and c be in G.P. with common ratio r, where $$a$$ $$\ne$$ 0 and 0 < r $$\le$$ $${1 \over 2}$$ . If 3$$a$$, 7b and 15c are the first three terms of an A.P., then the 4^th term of this A.P. is :