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

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