A fluid of dynamic viscosity $2 × 10^{−5}$ $kg/m.s$ and density $1$ $kg/m^3$ flows with an average velocity of $1$ $m/s$ through a long duct of rectangular ($25$ $mm$ × $15$ $mm$) cross-section. Assuming laminar flow, the pressure drop (in $Pa$) in the fully developed region per meter length of the duct is _______