Question 1077372
{{{F = P(1+r/n)^(nt) }}}

n = number of payments per year
r = interest rate per year, expressed as a decimal 
P = present value
F = future value
t = time, in units of years

This problem has: t=unknown, F=2P, r=0.08, n=1

{{{F = 2P = P(1+0.08)^(t) }}}
{{{     2 = (1.08)^t }}}
{{{  ln(2) = t*ln(1.08) }}}  <— key step, take ln() of both sides, {{{ ln(a^b) = b*ln(a) }}}
{{{   ln(2)/ln(1.08) = t }}}
{{{   highlight(t = 9.00years) }}}