Question

at what percentage per annum S.I will a sum double itself in 4 years​

Answer 1

Answer:Simple Thumb rule :

72/4 = 18%

Another way is to solve the equation :

2P = P(1 + r)^4

=> 2 = (1 + r)^4

Further calculation is not that simple except hit and trial method.

=> r = 0.1872

=> r = 18.72% ( approx )