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

*This problem is taken from the UKMT Mathematical Challenges.*