PIPES AND CISTERNS (capacity of cistern)

  daniel bro...
again you have to solve a problem for me
Q) Two pipes can fill a tank in 20 and 24 minutes respectively and a waste pipe can empty 3 gallons per minute.All the three pipes working together can fill the tank in 15 minutes.the capacity of the tank is..........

a)60 gallons
b)100 gallons
c)120 gallons
d)180 gallons

Hi , I would like to try it
as u know in 1 minute  pipe A and B can fill 1/20 and 1/24 respectively,and pipe C can empty 1/x  ,in the question it's also given that all the three working at a time can fill the tank in 15 minutes.
We can frame an equation 1/20 +1/24 -1/x=1/15 ,after solving it we get x=40   

pipe C can empty the tank at a rate of 3 gallons per minute so capacity of the tank is 40*3=120 gallons.

Thanks
It's all over to Daniel 

Hello Guys   Bibhu you posted the right answer buddy     great job! i hope Mohd's doubt is cleared now
The more questions we discuss the more we can learn