The ordinates of a one-hour unit hydrograph at sixty minute interval are $0, 3, 12, 8, 6, 3$ and $0 \: m^3/s$. A two-hour storm of $4$ cm excess rainfall occurred in the basin from $10$ AM. Considering constant base flow of $20 \: m^3/s$, the flow of the river (expressed in $m^3/s$) at $1$ PM is ________