Question

How long will it take for the balance of an account to double if the savings account earns 0.75% annual interest compounded monthly?

Hint: Set up the problem using the formula: A=P(1+r/n )^nt.

Answer 1

Let the amount deposited (principal) be x, then the amount after the required time = 2x.
A = P(1 + r/n)^nt: where A is the future value = 2x, P is the principal = x, r is the rate = 0.75%, n is the number of accumulation in a year = 12, t is the number of years.

2x = x(1 + 0.0075/12)^12t
2 = (1 + 0.000625)^12t
log 2 = 12t log (1.000625)
log 2 / log (1.000625) = 12t
1109.38 = 12t
t = 92 years