Rs. 4800 becomes Rs. 7200 at a certain rate of interest compounded annually in 5 years. Find the amount at the end of 15 years.

Rs. 16200

                Principal      :      Amount  

5yrs:           4800         :       7200

5yrs:               2          :           3

10yrs:            2²         :          3²

15yrs:            2³         :          3³

                      8          :         27

                      ⇓× 600             ⇓

                   4800                   ⇓× 600

                                           16200/-