A=future amount
P=present amount
r=rate in decimal
n=number of times per year compounded
t=time in years
how many years to double
basically
A=2P at t=?
so we can simplify and ignore the pricipal given and do
r=5.5%=0.055
n=1
t=t
take the ln of both sides
divide both sides by ln1.055
use calculator to aproximate
12.9462=t
about 12.9 years