The ordinates of a $2$-hour unit hydrograph for a catchment are given as
$$\begin{array}{llllll} \text{Time (h)} & 0 & 1 & 2 & 3 & 4 \\ \text{Ordinate } (m^3/s) & 0 & 5 & 12 & 25 & 41 \end{array}$$
The ordinate (in $m^3/s$) of a $4$-hour unit hydrograph for this catchment at the time of $3$ h would be _________