P$16-4$ A step tracer input was used on a real reactor with the

following results:

For t $<=10$ min, then C $=0$

For $10<=$ t $<=30$ min, then C $=10$ g$/$dm

For t $>=30$ min, then C $=40$ g$/$dm

$1.$ What is the mean residence time t $?$

$2.$ What is the variance $\backslash$sigma $?$