Rs. 2400 becomes Rs. 3600 at a certain rate of interest compounded annually in 5 years. Find the amount at the end of 15 years.

8100

                Principal      :      Amount  

5yrs:           2400         :       3600

5yrs:               2          :           3

10yrs:            2²         :          3²

15yrs:            2³         :          3³

                      8          :         27

                      ⇓× 300             ⇓

                   2400                   ⇓× 300

                                           8100/-