The sum of some terms of a G.P. is $315$ whose first term and the common ratio are $5$ and $2$ respectively. Find the last term and number of terms of the series.

Question

$\begin{array}{1 1}t_n=160 \; and \;n=6 \\t_n=320 \; and\; n=5 \\t_n=320 \;and \;n=6 \\ t_n=160\; and \;n=5 \end{array} $