If A can do a piece of work in n days then in 1 day’s work 
If A’s day’s work is 1/n than A can finish the work in n days.
Pipes & cisterns
Inlet- a pipe connected with a tank/cistern that fill it is known as an inlet.
Outlet- a pipe connected with a tank/cistern, emptying it is known as an outlet
1) If a pipe can fill a tank in x hours , then part filled in hour = 
2) If pipe can empty a full tank in y hour than part emptied in 1 hour= 
3) If a pipe can fill a tank in x hours & another pipe can empty the full tank in y hours. Than an opening both the pipe the net part filled/emptied
Where y<x filled in 1 hour = 
Where x>y emptied in 1 hour = 
part.