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1.  Inlet:
  A pipe connected with a tank or a cistern or a reservoir, that fills it, is known as an inlet.
  Outlet:
  A pipe connected with a tank or cistern or reservoir, emptying it, is known as an outlet.
2.  If a pipe can fill a tank in x hours, then:
 
part filled in 1 hour =
1
x
   
3.  If a pipe can empty a tank in y hours, then:
 
part emptied in 1 hour =
1
y
   
4.  If a pipe can fill a tank in x hours and another pipe can empty the full tank in y hours
(where y > x), then on opening both the pipes, then
 
the net part filled in 1 hour =  
1
x
-
1
y
     
5.  If a pipe can fill a tank in x hours and another pipe can empty the full tank in y hours
(where x > y), then on opening both the pipes, then
 
the net part emptied in 1 hour =  
1
y
-
1
x