The table below gives the frequency distribution of marks obtained by a group of students in a test. If the total mark scored is $200$, calculate the value of $y$

\text{Marks} & 3 & 4 & 5 & 6 & 7 & 8 \\ \hline \text{Frequency} & 5 & y-1 & y & 9 & 4 & 1 \\ \end{array}$$