Answer: Option 4
Net part filled in 1 hour
$$\eqalign{
& {\text{= }}\left( {\frac{1}{x} - \frac{1}{y}} \right) \cr
& = \left( {\frac{{y - x}}{{xy}}} \right) \cr} $$
∴ The tank will be filled in
$$ = \left( {\frac{{xy}}{{y - x}}} \right)\,\,{\text{hours}}$$