It takes $\mathrm{ab}$ man hours to paint $c$ square metres of the bridge and therefore $\frac{\mathrm{ab}}{c}$ man-hours to paint 1 square metre of the bridge. So $d$ men will take $\frac{\mathrm{ab}}{\mathrm{cd}}$ hours to paint 1 square metre and $\frac{\mathrm{abe}}{\mathrm{cd}}$ hours to paint $e$ square metres.