Oliver's grandfather invested $150 at 5% interest compounded quarterly. When Oliver inherited it, the amount was $6000. For how many years had the $150 been invested?

0.05/4=1.25% quarterly interest

6000=150 (1+0.0125) ^n, n = number of quarters

(1+0.0125) ^n=6000/150=40

n log (1.0125) = log 40

n=296.95 quarters

number of years=n/4=74.23 years.