two pipes a and b can seperately fill a tank in 20 mins and 30 mins respectively, which a third pipe C can empty the full tank iin 15 mins
If all pipes are opened together then in what time is the empty tank filled?
The second pipe fills the tank at a rate of 1/30 of a tank/min
The third pipe empties the tank at a rate of 1/15 of a tank/min.
Starting from empty the content of the tank at time t is:
The solution is the time t for which:
which is 60 minutes.
Though it seems to be the fashion of the day, I would discourage
learning rules at the expense of understanding. I normally work many
things, others seem to use rules for, from more primitive principles.