Answer
1.75.
Work Step by Step
Find the time for the beam traveling in air.
$$t_1=\frac{d}{c}=\frac{2.50m}{3.00\times10^8m/s }=8.33\times10^{-9}s $$
Find the time for the beam traveling in the block.
$$t_2=8.33\times10^{-9}s+6.25\times10^{-9}s=1.46\times10^{-8}s $$
The speed of light in the block is $\frac{2.50m}{1.46\times10^{-8}s }=1.71\times10^{8}m/s $
$n=\frac{c}{v}=\frac{3.00\times10^8m/s }{1.71\times10^8m/s }=1.75$.
