Question

A parent increases a child's allowance by 15% each year. If the allowance is $3 now, when will it reach $15?

Answer 1

t = no. of years for this to happen
:
3*(1.15^t) = 15
divide both sides by 3
1.15^t =
1.15^t = 5
:
use logs here
log(1.15^t) = log(5)
which is
t*log(1.15) = log(5)
:
t =log5/log1.15 ; enter into your calc
t ~ 11.5 years

Answer 2

15% increrase means: in one year you will have \((100\%+15\%)=115\%\) of the original amount

115%=1.15