# HELP WITH DOUBLING FUNCTION D(t)!!!!

• Mar 17th 2008, 05:56 PM
Buckeye00
HELP WITH DOUBLING FUNCTION D(t)!!!!
I am stuck on this prolbem..if someone can explain it to me better or just guide me through it that would be great!..Thanks for your help!

1. The doubling function D(t) 1n(2)/1n(1+r) gives the number of years required to double the amount of money invested at the interest rate r (expressed as a decimal), compounded annually. How long will it take to double your money at an interest rate of 3.25%?

2. How long will it take to double your money at an interest rate of 5%?

3. What interest rate would you need to double yoru money in 11 years?
• Mar 17th 2008, 06:01 PM
topher0805
Your equation is a bit confusing... Is this what you meant?

$D(t) = \frac {ln 2}{ln (1+r)}$
• Mar 17th 2008, 06:06 PM
Buckeye00
Yes that is correct

Sorry..I just got singed up with this website..and I don't know how to type in fractons like that
• Mar 17th 2008, 06:11 PM
topher0805
Ok, this equation does not make any sense. D(t) implies that D is dependent on the variable t, but there is no t in the equation.

Are you sure that you didn't meant this?:

$
T(r) = \frac {ln 2}{ln (1+r)}
$
• Mar 17th 2008, 06:18 PM
Buckeye00
my bad..yes I did mean that.
• Mar 17th 2008, 06:34 PM
topher0805
For the first two questions simply plug in 0.0325 and 0.05 for r to get your answers.

For the third question, set it equal to 11 and solve for r:

$
11=\frac {ln 2}{ln (1+r)}
$

Solve for r...