Rs.320 becomes Rs.480 at a certain rate of interest compounded annually in 4 years. Find the amount at the end of 16 years.

1620

                Principal      :      Amount  

4yrs:            320         :       480

4yrs:              2          :           3

8yrs:             2²         :          3²

12yrs:            2³         :          3³

16yrs:            2⁴         :          3⁴

                    16          :       81

                      ⇓× 20             ⇓

                   320                   ⇓× 20

                                           1620/-