Math and Arithmetic

Physics

Brain Teasers and Logic Puzzles

# How much time will it take for two pipes that are opened at the same time to fill a tank if one will fill it when opened by itself in 36 hours and the other will fill it in 45 hours?

###### Wiki User

###### October 03, 2007 4:41PM

The first pipe will fill one tank in 36 hours. Therefore in one hour it will fill 1/36 tanks. The second pipe will fill one tank in 45 hours. Hence in one hour it will fill 1/45 tanks. Between them, then, the two pipes will fill 1/36 + 1/45 tanks in an hour. This equals 5/180 + 4/180 = 9/180 = 1/20 tanks per hour. Hours per tank is just the reciprocal of tanks per hour, i.e. 1/[1/20] = 20 hours to jointly fill a tank. Incidentally, adding reciprocals and taking an overall reciprocal is a very commonly used trick. It is used to calculate net parallel resistance in electric circuits. Here, time to fill a tank (or resistance to tank-filling!) can be considered analogous to electrical resistance.

## Related Questions

###### Asked in Math and Arithmetic

### 9 large pipes drains a pond in 8 hours and 6 small pipes drains a pond in 16 hours How long will 3 large pipes and 5 small pipes take to drain a pond?

If 9 large pipes take 8 hours to drain the pond,
then 1 large pipe would take 8*9=72 hours.
large_pipe_rate = (1/72) pond/hour
Since 6 small pipe drain the pond in 16 hours,
then 1 small pipe would take 6*16 = 96 hours.
small_pipe_rate = (1/96) pond/hour
Now we can calculate using:
rate * time = work done
In this case, we have two rates, but a common time,
so we will have:
(3*large_pipe_rate)*time + (5*small_pipe_rate)*time = 1
Plug in the values we know and solve for "time":
(3*large_pipe_rate)*time + (5*small_pipe_rate)*time = 1
(3 * (1/72))*time + (5 * (1/96))*time = 1
(3/72)*time + (5/96)*time = 1
(3/72 + 5/96)*time = 1
time = 1/(3/72 + 5/96)
time = 1/.09375000
time = 10.66666667
Answer:
3 large pipes and 5 small pipes could drain
the pond in 10 and 2/3 hours (which is
10 hours and 40 minutes).
-----------------------
The other answers disagree with me. I would like you to
get this problem correct, and to be honest, I'd like the
points for showing the correct way to do it. So, I'll take
a moment to show you why the other answers don't
even make sense.
Consider if all 8 pipes were large. We know that *9*
large pipes could drain the pond in 8 hours. With a
little calculation, we can see that 8 large pipes could
drain it in 9 hours:
8 * (1/72) * time = 1
(1/9) * time = 1
time = 9 hours
If all 8 pipes were small, then it would take:
8 * (1/96) * time = 1
(1/12)* time = 1
time = 12 hours
So we **know** it will be somewhere between 9
and 12 hours. Certainly NOT 16 hours.
Go back to the problem statement and notice that 6
small pipes can drain the pond in 16 hours. The other
answers claim that 5 small pipes + 3 large pipes also
take 16 hours. Therefore, their claim is that:
6 small pipes = 5 small pipes + 3 large pipes
In other words, by removing 1 of the 6 small pipes and
adding 3 large ones, the drain time remains at 16 hours:
1 small pipe = 3 large pipes
Nonsense.
Bottom line:
3 large + 5 small will take 10 hours and 40 minutes.

###### Asked in Antiques

### Where can you find Raw diamonds?

###### Asked in Physics

### What happens when pipes freeze?

###### Asked in Operating Systems

### Give an example of a situation in which ordinary pipes are more suitable than named pipes and an example of a situation in which named pipes are more suitable than ordinary pipes?

Named pipes can be used to listen to requests from other
processes( similar to TCP IP ports). If the calling processes are
aware of the name, they can send requests to this. Unnamed pipes
cannot be used for this purpose.
Ordinary pipes are useful in situations where the
communication needs to happen only between two specified process,
known beforehand. Named pipes in such a scenario would involve too
much of an overhead in such a scenario.

###### Asked in Swimming Pools, Pool Care and Cleaning, Physics

### What temp does pool pipes freeze?

Water freezes at 32 degrees Fahrenheit (which is 0 degrees
Celsius).
The factors which might change the temperature at which your
pool pipes may freeze are:
- other materials (such as salt) which are dissolved in the
water
- the rate of flow (if any) of the water through the pipes
- the pipe diameter
- the insulating properties of the pipes themselves
- whether the pipes are buried underground
Good luck with your pool!