A swimming pool can be filled by pipe A in 3 hours and by pipe B in 6 hours, each pump working on its own. At 9 am, pump A is started. At what time will the swimming pool be filled if pump B is started at 10 am?

Let pool volume be 30 litres.

Flow rate through A = 30/3 = 10 litres/hr

Flow rate through B = 30/6 = 5 litres/hr

A is opened at 9:00am and then B is opened at 10:00 am. So, A alone fills the tank for an hour to 10 litres.

After 10:00am both are opened simultaneously.

Remaining 20 litres of the pool are filled at a combined flow rate of 15 litres/hr which takes 80 min to get filled.

$$\therefore\ $$ The pool is filled 80 mins after 10:00am i.e. 11:20am