Statistics

If the mean and the variance of the data

$$ \begin{array}{|c|c|c|c|c|} \hline \text { Class } & 4-8 & 8-12 & 12-16 & 16-20 \\ \hline \text { Frequency } & 3 & \lambda & 4 & 7 \\ \hline \end{array} $$

are $\mu$ and 19 respectively, then the value of $\lambda+\mu$ is :