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?