A loan earns interest at a certain compound rate per annum. Four years ago the amount was Rs 81, now it is Rs 144. What will the amount be two years hence? Find what time a sum of money will double itself at 4.5% interest compounded annually.

In what time will a sum of money double itself at 5% p.a. compound interest? [Given log 2 = 0.3010, log105 = 2.0212]