If tanisha has $100 to invest at 8% annum compounded monthly, how long will it be before she has $150?

An exponential behavior can be observed.

Let x be the number of years she has to wait.

100 ((1+0.08/12) ^12x) = 150

Now solve for x.

log base 1.08/12 of 1.5 = 61.02 approx

12x=61.02

x=5.09 years