Residence time distribution in a CSTR operating at steady state is studied by introducing pulse input of tracer in the feed stream. The concentration of the tracer in the exist stream at various time intervals are given in table. The time is measured relative to that at which the tracer is injected.

$\backslash$table$[[\backslash$table$[[$t seconds$)]],0,6,12,60,120,300,600,1800],[\backslash$table$[[$tracer$],[($mg$/$lit$)]],0,0\mathrm{.}17,0\mathrm{.}17,0\mathrm{.}15,0\mathrm{.}125,0\mathrm{.}07,0\mathrm{.}02,0\mathrm{.}001]]$

Calculate: $($i$)$ the average residence time $($ii$)$ the dispersion number.