Question 1087136
f = p * (1 + r) ^ n


f is the future value
p is the present value
r is the interest rate per time period
n is the number of time periods.


in your problem:


f = 1331
p = 1000
r = .1 per year
n = number of years


the formula becomes:


1331 = 1000 * (1 + .1) ^ n


divide both sides of the equation by 1000 and simplify to get:


1.331 = 1.1 ^ n


take the log of both sides of the equation to get:


log(1.331) = log(1.1 ^ n)


simplify to get log(1.331) = n * log(1.1)


solve for n to get n = log(1.331) / log(1.1) = 3


1000 rupees will become 1331 rupees in 3 years at an interest rate of 10% compounded annually.


in general, log(b^n) = n*log(b)


that's one of the basic properties of logarithms.