Exponential functions? NOO

[SaYiaN]

Damn what the hell. Why can't i solve this.

Determine how much time is required for an investment to double in value if interest is earned at the rate of 6.25% compounded monthly.

I'm using formula

a = ao^kt

k being constant
t being time
ao being inital 
a being total

[enoughfakefiles]

/moved to homeworkworld.

[Barbarossa]

FV = PV(1 + i/m)^mt

FV - future value
PV - present value
i - rate of interest
m - number of times per year interest is compounded
t - number of years until maturity of the investment

So in your case, you need to find t. But you know that FV = 2PV,

so:

2PV = PV(1+0.0625/12)^12t
2=(1+0.0625/12)^12t
12th-root-of-2 = 1.0052^t
1.0594 = 1.0052^t

So t = just under 11.14 years.