A tank has a capacity of 120 litre and has two inlets (taps) A and B. When both the inlets were opened simultaneously for 3 hours, they were able to fill 35 litres water in the tank. If inlet A alone can fill the empty tank completely in 24 hours, how many hours will it take to fill the empty tank when inlet B is opened alone ?

∵ Inlet A fills the empty tank in 24 hours.

∴ Quantity of water filled by pipe

A in 1 hour = $frac{{120}}{{24}} = 5$ litres

Quantity of water filled in the tank by pipes A and B in 3 hours = 35 litres

∴ Quantity of water filled by both pipes in 1 hour = $frac{{35}}{3}$ litres

∴ Quantity of water filled by pipe B in 1 hour

$= left( {frac{{35}}{3} - 5} ight)$ litres$= left( {frac{{35 - 15}}{3}} ight)$ litres$= frac{{20}}{3}$ litres${ m{Required;time}} = left( {frac{{120}}{{frac{{20}}{3}}}} ight)$ Hours$= left( {frac{{120 imes 3}}{{20}}} ight) = 18$ Hours