Explanation
Step 1
find the speed of ligth in the diamond:
to do that we can use the formula
[tex]refractive\text{ index\lparen}\mu)=\frac{c(\text{ speed of light in vaccum\rparen}}{s(\text{ speed of light in the material\rparen}}[/tex]we know that :
[tex]\begin{gathered} refractive\text{ index of diamond\lparen}\mu)=2.42 \\ speed\text{ of llight in the vaccum=c=300000 }\frac{km}{s} \end{gathered}[/tex]so, replace and solve for speed of light in the diamond
[tex]\begin{gathered} refractive\text{ index\lparen}\mu)=\frac{c(\text{ speed of light in vaccum\rparen}}{s(\text{ speed of light in the material\rparen}} \\ 2.42=\frac{3*10^5\frac{km}{s}}{s} \\ s=\frac{3\times10^5\frac{km}{s}}{2.42} \\ s=123966.9\text{ }\frac{km}{s} \\ \end{gathered}[/tex]finally, to find the percentage apply this formula
[tex]\begin{gathered} \text{ \% }=\frac{speed\text{ of light in diamond}}{spped\text{ of ligth invaccum}}*100\text{ \%} \\ replace \\ \text{ \%=}\frac{123966.9\frac{km}{s}}{3*10^5\frac{km}{s}}*100\text{ \%} \\ \text{ \%=0.413*100\%} \\ \text{ \%=41.3 \%} \\ rounded\text{ 41\%} \end{gathered}[/tex]so, the answer is
41 %
I hope this helps you