Question

Manny has his money in a savings account earning 3 percent interest. How long will it take his money to double?

Answer 1

I will assume monthly compound interest.

Then:

yearly interest: 12%
monthly interest = (3/12) % 
invested money: C
Final money: 2C
n: number of months

Compund interest formula:

Final money = Invested money * (1 + 0.03/12)^n

2C = C * (1.0025)^n

2C/C = (1.0025)^n

2 = (1.0025)^n

n = log(2) / log (1.0025)

n = 277.6 months

Time = 278 months = 23.1 years