ANSWER
[tex]\begin{equation*} 2.92 \end{equation*}[/tex]EXPLANATION
To find the index of refraction of the liquid, we have to apply the formula for critical angle:
[tex]\theta=\sin^{-1}(\frac{n_r}{n_i})[/tex]where nr = refractive index of air = 1
ni = refractive index of liquid
Hence, by substituting the given values into the equation, we have that the index of refraction of the liquid is:
[tex]\begin{gathered} 20=\sin^{-1}(\frac{1}{n_i}) \\ \sin20=\frac{1}{n_i} \\ n_i=\frac{1}{\sin20} \\ n_i=2.92 \end{gathered}[/tex]That is the answer.
