If you deposit money today in an account that pays 6.5% annual interest, how long will it take to double your money

deposit is compounded monthly.

D = deposit
y = number of years deposited
t = number of times compounded annually
A = 6.5 %
r = A / t
ty = number of periods compounded
D ( 1 + r ) ty = 2D

   (1 + r )ty   = 2

ln ((1 + r )ty) = ln (2)

ty ln(1 + r ) = ln (2 )

ty = [ln(2)] / [ln (1 + r)]

ty = [ ln(2) ] / [ ln ( 1 +( 0.065/12) )]

ty = 128.31

It will take 128.31 months for you deposit to double if compounded monthly.
After 129 months you will more than double your deposit.