Which term of the G.P. 3, 6, 12, 24,….. is 768?

Which term of the G.P. 5, 10, 20, 40,…… is 640?

Find the G.P. whose 5th and 8th terms are 80 and 640 respectively.

The 4th term of a G.P. is 16 and the 7th term is 128. Find the first term and common ratio of the series.