Pipe A can fill in 20 minutes and Pipe B in 30 minutes and pipe C can dump the same in 40 mins.If all of them work together , find the time it takes to fill the tank

hey guys thanks for answering so far..just want to clarify that by Dump I mean that Pipe C can EMPTY the tank in 40 mins
Here s a suggestion: take out there rates per minute (1/20, 1/30, 1/40) and then add rate of pipe a and pipe b and subtract pipe c.


The work done by A and B per minute=1/20+1/30=5/60=1/12
work done by C per minute=1/40
if all work together=1/12-1/40=7/120
so time taken if all work together is 120/7=17.14 mins
now u know
that 2 people had ansered it incrrtly.
Time taken by A=20 min
Time taken by B=30 min
Time taken by C(to empty)=40 min
C's work done is -ve.
Work done by all of them in one hour=1/20+1/30-1/40
Therefore, They will take 120/7 min= 17 1/7 min to fill the tank.