Two identical taps fill 2/5 of a tank in 20 minutes

  • Thread starter Thread starter mgb_phys
  • Start date Start date
  • Tags Tags
    Tank
Click For Summary
SUMMARY

The forum discussion centers on a mathematical problem involving two identical taps filling a tank. The problem states that the taps together fill 2/5 of the tank in 20 minutes, leading to various interpretations of how long it would take for one tap to fill the remaining portion once the other tap goes dry. The consensus suggests that if both taps fill 2/5 of the tank in 20 minutes, one tap would take 60 minutes to fill the remaining 60%. However, if each tap fills 2/5 individually in the same time, one tap would take only 10 minutes to fill the remaining 20%. The 'correct' answer is debated to be 15 minutes under certain assumptions. The discussion highlights the ambiguity of the question and its implications for problem-solving in real-world scenarios.

PREREQUISITES
  • Understanding of basic algebra and rates of work
  • Familiarity with the concept of flow rates in fluid dynamics
  • Knowledge of problem-solving techniques in mathematics
  • Ability to interpret ambiguous questions
NEXT STEPS
  • Study the principles of rates and work problems in algebra
  • Learn about flow rate calculations in fluid dynamics
  • Explore common mathematical riddles and their solutions
  • Investigate the implications of ambiguous questions in professional settings
USEFUL FOR

This discussion is beneficial for mathematicians, educators, students preparing for standardized tests, and professionals involved in problem-solving and critical thinking exercises.

mgb_phys
Science Advisor
Homework Helper
Messages
7,901
Reaction score
15
So a particularly dumb client has decided that before I can work for them I have to pass a high school maths test.
Never having had to do US school tests I looked at some practice questions, including:

"Two identical taps fill 2/5 of a tank in 20 minutes. When one of the taps goes dry in how many minutes will the remaining one tap fill the rest of the tank ?"

Brilliantly ambiguous question
If they mean that two taps together have filled 2/5 (40%) of the tank in 20mins, then each fills 20% in 20mins, ie 1%/min and so one will take 60mins to fill the remaining 60%.

If they mean that each of the taps alone fill 2/5 (40%) in 20mins, then each fills 2%/min and then one will fill the remaining 20% in 10min.

Or if you assume they are connected to the same water supply so that when one stops the other doubles in flow then you have either 30mins or 5mins.

The 'correct' answer apparently is 15mins?
 
Physics news on Phys.org
mgb_phys said:
So a particularly dumb client has decided that before I can work for them I have to pass a high school maths test.
Never having had to do US school tests I looked at some practice questions, including:

"Two identical taps fill 2/5 of a tank in 20 minutes. When one of the taps goes dry in how many minutes will the remaining one tap fill the rest of the tank ?"

Brilliantly ambiguous question
If they mean that two taps together have filled 2/5 (40%) of the tank in 20mins, then each fills 20% in 20mins, ie 1%/min and so one will take 60mins to fill the remaining 60%.

If they mean that each of the taps alone fill 2/5 (40%) in 20mins, then each fills 2%/min and then one will fill the remaining 20% in 10min.

Or if you assume they are connected to the same water supply so that when one stops the other doubles in flow then you have either 30mins or 5mins.

The 'correct' answer apparently is 15mins?

I also like how a single tap can fill 3/5 of a tank faster (in allegedly 15 minutes) than two taps filling up only 2/5 of a tanks (20 minutes).

You might be better of not working with said client... I'd hate for them to claim they paid you enough when in fact, they had not :S -- and their contract is as ambiguous as the problem.
 
No wonder why maths is hard!
 
maybe its the average of all your different answers ;)
 
Apparently this is test (GMAT) used by US schools to decide who does an MBA!
That would certainly explain a lot about the recent events on wall St.
 
Is this question anyway related to Bernoulli's principle
 
snshusat161 said:
Is this question anyway related to Bernoulli's principle

Bernoulli's wouldn't have too great an effect on the system; it's water.
 
but I've read that bernoulli's theorem works best with low viscous liquid like water.
 
Since it's a multiple choice question supposedly aimed at testing the mental arithmetic of MBA students I'm guessing that non-linear computation fluid dynamics effects aren't a major consideration.

However I'm guessing that idiots setting this are basing their selection on "the computer says C so that mist be right - it's a computer".

If anyone has read Neal Stephenson's Cryptomnicon there is a brilliant example of this. The hero joins the navy and is set a simple ferryman problem as an IQ test - he assumes it can't be that simple and spends the entire exam developing a totally new theory of calculus of fluid flows.
Because he didn't answer any question he is graded F and sent to play in a band, arriving at Pearl Harbor just as his paper appears in the maths journal.
 
  • #10
Since it's a multiple choice question supposedly aimed at testing the mental arithmetic of MBA students I'm guessing that non-linear computation fluid dynamics effects aren't a major consideration.

However I'm guessing that idiots setting this are basing their selection on "the computer says C so that mist be right - it's a computer".

If anyone has read Neal Stephenson's Cryptomnicon there is a brilliant example of this. The hero joins the navy and is set a simple ferryman problem as an IQ test - he assumes it can't be that simple and spends the entire exam developing a totally new theory of calculus of fluid flows.
Because he didn't answer any question he is graded F and sent to play in a band, arriving at Pearl Harbor just as his paper appears in the maths journal.

Yeah. In my view you are very correct but I'm surprised that how none of them can figure it out. It's not only your book but I've seen this question with the same answer You've told in many websites, even those tutorial websites which have a very high reputation.

But for all those who wants to have some exercise of their brain, I've a question for You. Try to solve this question without using any knowledge of Physics. You'll find it very interesting.

"Three students are standing in a queue. A person came with a box containing two black hats and three white hats. He randomly put hats on their head and asked the students not to turn back neither to look up for the hat color. Now he started asking from the guy at the back for the color of the hat on his head but he can’t answer, then he asked from the second but he also can’t answer and when he asked from the next student (who was in the front of the queue), he answered correctly with complete explanation. Now think you are that student and I’m expecting the same answer from You."
 
  • #11
white
 
  • #12
used probability? Tell me how you've made this conclusion.
 
  • #13
3rd guy doesn't know what he is, so both in front can't have B (black), giving options:
1st W W B
2nd W B W
3rd ?
If 1st was B, then 2nd would know he is W... means 1st guy must be W
 
  • #14
lanedance, You really have a very high IQ level. I took about half an hour of constant thinking to solve this problem. How much time you took? According to my estimate you have solved this riddle in less than 5 minutes. Do you have heard this riddle before I've posted?
 
  • #15
snshusat161 said:
lanedance, You really have a very high IQ level. I took about half an hour of constant thinking to solve this problem. How much time you took? According to my estimate you have solved this riddle in less than 5 minutes. Do you have heard this riddle before I've posted?

I've heard of another variation to this.
But as future reference, if someone is giving you a riddle to solve, surely they would have heard about it and solved it beforehand.
 
  • #16
mgb_phys said:
If they mean that two taps together have filled 2/5 (40%) of the tank in 20mins, then each fills 20% in 20mins, ie 1%/min and so one will take 60mins to fill the remaining 60%.

My guess is that they meant this and accidentally divided by 2 instead of multiplying.
 

Similar threads

Fluid dynamics problem: Equalize the volume of water in tanks at differing heights
  • · Replies 50 ·
2
Replies
50
Views
7K
How Long to Fill Tank with 3 Pumps?
  • · Replies 2 ·
Replies
2
Views
2K
Lawn/Garden Cycling water, nutrients in pots, garden beds, containers, hydroponics
  • · Replies 2 ·
Replies
2
Views
3K
Efficient Pump Usage in Fuel Oil Supply System: Solving for Tank Fill Time
  • · Replies 6 ·
Replies
6
Views
1K
Solve Time for Tank Filling with 2 Pipes
  • · Replies 2 ·
Replies
2
Views
4K
Trouble solving for end state of two control volumes in a rigid tank
  • · Replies 12 ·
Replies
12
Views
2K
Braking spacecraft in a snow-filled tank?
  • · Replies 7 ·
Replies
7
Views
3K
System of diff eqs modeling tanks of brine
  • · Replies 11 ·
Replies
11
Views
3K
Ideal gas expansion between two tanks (Thermodynamics)
  • · Replies 9 ·
Replies
9
Views
11K
Graduate Why do tanks get hot when you fill them from higher pressure tanks?
  • · Replies 13 ·
Replies
13
Views
12K