# help me anyone

• Sep 16th 2006, 01:01 AM
gautam93
help me anyone
hi]

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?

bye
gautam93
• Sep 16th 2006, 01:28 AM
CaptainBlack
Quote:

Originally Posted by gautam93
hi]

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?

bye
gautam93

The first pipe fills the tank at a rate of 1/20 of a tank/min
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:

t(1/20+1/30-1/15).

The solution is the time t for which:

t(1/20+1/30-1/15)=1,

which is 60 minutes.

RonL
• Sep 16th 2006, 05:02 PM
ThePerfectHacker
Here is a useful rule I develpoed that seems to help people.

If it takes 'a' units of time to do a job and 'b' units of time to do the same job by two different things or people then it takes (ab)/(a+b) to do it together.
• Sep 17th 2006, 12:51 AM
CaptainBlack
Quote:

Originally Posted by ThePerfectHacker
Here is a useful rule I develpoed that seems to help people.

If it takes 'a' units of time to do a job and 'b' units of time to do the same job by two different things or people then it takes (ab)/(a+b) to do it together.

Your rule does not lead to understanding, while the reason for it does.

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.

RonL