Answer
$t=7^{th} year$
Work Step by Step
The formula for compound interest for $n$ compounding per year is, $P(1+\frac{r}{n})^{nt}$, whereas $P$ is the initial investment, $r$ is rate, $t$ is time period, $n$ compounding period.
$12500(1+\frac{0.065}{4})^{4t} = 20000$,
$(1.01625)^{4t} = 1.6$,
$4t \log (1.01625) = \log (1.6)$,
$t = \frac{\log (1.6)}{4 \log (1.01625)}=7.28942$,
$t=7.3$
