Question 274777
A parent raises a child’s allowance by 15% each year. If the allowance
is $3 now, when will it reach $15? 
:
How about this?
t = no. of years for this to happen
:
3*(1.15^t) = 15
divide both sides by 3
1.15^t = {{{15/3}}}
1.15^t = 5
:
use logs here
log(1.15^t) = log(5)
which is
t*log(1.15) = log(5)
:
t = {{{log(5)/log(1.15)}}}; enter into your calc
t ~ 11.5 years, which is what you said