Answer
$22.3ns$
Work Step by Step
We know that
$d=vt$
$\implies d=\frac{c}{n}t$
This simplifies to:
$t=\frac{nd}{c}$.......eq(1)
For the first medium, the above equation can be written as
$t_1=\frac{n_1 d_1}{c}$
We plug in the known values to obtain:
$t_1=\frac{1.51(1.51)}{3.00\times 10^8}=7.60ns$
Now for the second medium, eq(1) can be written as
$t_2=\frac{n_2d_2}{c}$
We plug in the known values to obtain:
$t_2=\frac{1.33(3.31)}{3.00\times 10^8}$
$t_2=14.67ns$
Now the total time is $t_{total}=t_1+t_2=14.67+7.60=22.3ns$
