Question

An empty tank be filled by pipe A in 4 hours &by pipe B in 6 hours. Ifthe two pipe are opened opening pipe A ,then the tank will be filled in .

Answer 1

First pipe can fill 1/4 in one hour
Second pipe can fill 1/6 in one hour. Two pipes can together can fill 1/4+1/6=5/12 in one hour
Two pipes can fill the full tank in 12/5hours

Answer 2

2 hour's to fill the tank is needed