A draining pipe can empty a pool in 4 hours. On a rainy day, when the pool is full, the draining pipe is opened and the pool is emptied in 6 hours. If rain inflow into the pool is 3 liters per hour, what is the capacity of the pool?

A. \(9\) liters
B. \(18\) liters
C. \(27\) liters
D. \(36\) liters
E. \(45\) liters
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Official Solution:

A draining pipe can empty a pool in 4 hours. On a rainy day, when the pool is full, the draining pipe is opened and the pool is emptied in 6 hours. If rain inflow into the pool is 3 liters per hour, what is the capacity of the pool?

A. \(9\) liters
B. \(18\) liters
C. \(27\) liters
D. \(36\) liters
E. \(45\) liters


Let the rate of the draining pipe be \(x\) liters per hour. Then the capacity of the tank will be \(C=time*rate=4x\);

Now, when raining, the net outflow is \(x-3\) liters per hour, and we are told that at this new rate the pool is emptied in 6 hours. So, the capacity (C) of the pool also equals to \(C=time*rate=6(x-3)\);

Thus we have: \(4x=6(x-3)\). Solving gives \(x=9\). Therefore \(C=4x=36\).


Answer: D
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other method
let x be the total capacity
x/4 equal to the drain rate. 3 equals rain inflow
6 hours rain inflow equals 18
x/4=(x+18)/6
6x=4x+72
2x=72
x=36

Here's how I approached it.

Rain inflow is 3l/hr
After 6 hours it would have filled in 18l
On top of this amount, the pipe would also have to drain the contents of the already filled pool.

We know that draining a filled pool takes 4 hours.
But draining a filled pool + the 18l takes 6 hours, hence it takes 2 hours to drain 18l.

Then do a ratio comparison:
If 18l takes 2 hrs, a filled pool takes 4 hrs.
18:2 = filled pool:4

Hence, filled pool = 36l

It's also easier to just draw a diagram that would look something like this:

|__+18___| 2 hrs
|      x      | 4 hrs
|________|

From the diagram, it is more obvious that x is 36.

This is how I solved it.

Recall rate eqn: 1/r = 1/r1 + 1/r2

In this case, the rate is negative, hence, 1/r = 1/r1 - 1/r2

i.e. 1/6 = 1/4 - 1/r2

1/r2 = 1/4 - 1/6 = 1/12

Hence r2 = 12

W = RT
C = RT = 3 * 12 = 36
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May I know why you considered rate to be negative, please? Any examples or resources to that extent is appreciated.

Instead of solving question with conventional method, solve it by plugging in the answer choices and out of the given answer choices 36 is the best to pick up.

I appreciate all the methods mentioned but they involved a 2-step equation process.

I used this one involving only the rate equation :

Capacity of the pool : x litres
Draining pipe rate : x/4 --- x litres per 4 hours
Rain inflow rate : 3/1 --- 3 litres per 1 hour (negative in this case as draining is emptying and rain is filling the pool)
Group rate : x/6 --- x litres per 6 hours

Usual rate equation : 1/r1 + 1/r2 = 1/r ===> x/4 - 3/1 = x/6 ===> solve for x.

I did it by calculating the reduction in rate, caused by the rain.

So, normally the drain pipe drains the full pool in 4 hours. This means that the draining rate is 1/4.
That rainy day, because the rain was filling the pool while the drain was emptying it, the pool was drained in 6 hours. This means that the draining rate was 1/6.

Now, 1/4-1/6 = 1/12. This must be the rate of the rain inflow. This means that the pool will be totally full in 12 hours (only by the rain).

So, 3 liters per hour is 3*12 = 36. Sounds complicated even to me that I solved it in this way, but sometimes it happens... So, is this way indeed correct?

If you follow % efficiency method it will solved in seconds. The outlet pipe can drain the water in 4 days which mean its efficiency is 25% (divide 100 by 4) and when it rains its efficieny is reduced to 16.66% (divide 100 by 6) that means the rain is filling the pool at an efficiency of 8.33% (25%-16.66%). Now from the calculation we can deduce that the rain will take 12 hours to fill the pool (100/8.33%). So just multiply 12 with 3 and get the answer. This approach saves time but one needs to memorise the percentage to fraction conversion table. For example 12.5% =1/8.

Tank Capacity=x
Drain rate=x/4 l/hr
Filling rate=3l/hr
Capacity of Tank in 6 hrs= 0
0=x + (3*6) - 6*(x/4)
x=36 litres

My 2 cents:


If you have any question, please let me know & i do like kudos

Alternate approach:

Normal Time = 4 hours
Because of rain the pipe had to work for extra 2 hours.

Extra inflow to the pool = 3 * 6 = 18 litres

The pipe took 2 hours extra to drain out the extra 18 litres --> Rate = 18/2 = 9lt/hour

Total capacity = 9 * 4 = 36

Answer: D

L/4 is drain rate.

for six hours it is 6*L/4

Total water drained in siz hours is 6*3 + L

Therefore, 18 + L = 6*L/4

=> L = 36 which is capacity of Tank

I solved backwards from the answers:
B - 18 L / 4 hours = 4.5 L per hour drained. 4.5 * 6 hours total to drain = 27 liters total were drained. Subtract 18 (6 hours * 3 Liters per hour of rain) and you get 9. 9 does not equal 18 (because the pool started full), so try D.
D - 36 L / 4 hours = 9 L per hour drained. 9 * 6 = 54 liters total were drained. Subtract 18 and you get 36. This is the correct answer.

Hours to drain with no rain = 4 hrs
Hours to drain with rain = 6 hrs
Hours to drain water from rain = 2 hrs
Water from rain = 3L/hr * 6hrs = 18L
Drain rate (I used only rain water) = 18L/2hrs = 9L/hr
Capacity (I used without rain) = 4hours * 9L/hr = 36L

Its a simple question, quite GMAT Like

very easy to mess up in haste
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Here's my approach, which may be less than efficient, but going through the logic of it like this helped me not fall into the trap of mindlessly trying W=RT.

First, we are given that we have a pool. Let's call the capacity of the pool X Liters. We know that the drain pipe can drain X Liters in 4 hours. We can represent this as X/4.

Next we learn that when some additional liters are added to the pool, it now takes the drainpipe 6 hours to drain the pool. How many additional liters were added? We are given that 3L of water per hour was added (the rain), and this went on for 6 hours, so we can determine that 6*3 = 18L of water was added to the pool.

From this we can derive our next rate: X liters + 18 L now takes 6 hours to drain. We represent this as (X + 18)/6.

We now have two rates, which we can set equal to one another. Remember that X is the capacity of the pool.

X/4=(x+18)/6
X=36.

We don't need any equations -- we just need to think logically here.

Rain inflow is 3 liters per hour * 6 hours = 18 liters.

This 18 liters increased the time it took to empty the pool by 2 hours. So if we're told a draining pipe can empty a pool in 4 hours, we can simply multiply 18 liters by two:

18 liters * 2 (for 4 hours) = 36 liters
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