Now the total interest for 42 months will be:-
[tex]\begin{gathered} S\mathrm{}I\mathrm{}=\frac{9000\times13\times7}{200} \\ =\frac{90\times13\times7}{2} \\ =45\times13\times17 \\ =4095 \end{gathered}[/tex]So the total amount he has to pay after 42 months will be = 9000+4095
= $13095
So
[tex]\begin{gathered} 42\text{ months = \$13095} \\ 1\text{ month =}\frac{13095}{42} \\ =311.79 \end{gathered}[/tex]So his monthly installment will be $ 311.79
So $ 311.79 is the correct option.