Question 932413
Almost done with the equation formulation.  One doubling period is {{{18/12=3/2}}} years.


{{{n*periods*(1&1/2)*(years/period)=t*years}}}, introducing the new variable, t years.  This means, {{{n(1&1/2)=t}}}, and then solving for n,  {{{n=(2t/3)}}}.


Now you can use the equation model, {{{highlight(p=10*2^(2t/3))}}}.



This model with the use of t years is NOT the only way to handle any specific question.  You could also convert the 9 years requested into how many doubling periods and use your original model.
{{{9*years*(1/(3/2))*(periods/years)=9*3/2=27/2=13&1/2}}}, doubling periods.