Pipe and Tank Question

[Hobbiez]

If Pipe A that can fill a tank to its brim in 5minutes. Another pipe, Pipe B took 7minutes to empty the full tank.

 

If pipe A is filling up and pipe b is emptying at same time, how long will it take to fill up the tank?

 

Thanks for any advice.

Edited May 7, 2008 by Hobbiez

[Booboon]

sibei chim sia. Any maths teacher around?

[KapitanE]

This question, we have to do some assumption as the volume of the tank is not stated in the question. Let us then assume that the volumeis 10 litres.

 

If it takes 5 minutes to fill the tank, then it will be filling at a rate of 2 litres a minute i.e. 10 litres / 5 minutes.

 

If it takes 7 minutes to empty the tank, the rate of discharge will be 10 litres / 7 minutes = 1.44 litres per minute.

 

Therefore, the difference between filling and loading is 0.56 litres per minute. So to fill 10 litres tank at 0.56 litres per minute will be......

 

17.9 miuntes or 17 minutes and 52 seconds.....

[Shinshino]

My take.

 

Let volume of tank = V

Flowrate thru pipe A = FR_A

Flowrate thru pipe B = FR_B

 

Basic equation, Volume [m^3] = Flowrate [m^3/min] * Time [min]

Units added for illustration.

 

FR_A = V / 5

FR_B = V / 7

 

Net flow rate into tank = FR_A

[Mivec9]

Let V be volume of tank.

 

Since total flow rate = Flow rate of Pipe A - Flow rate of Pipe B

(t is the time we need to find)

 

V/t = V/5 - V/7

V/t = 2V/35

t = 35/2 = 17.5min

[Hobbiez]

Thanks for all answers. The last three solutions have correct answers.

Edited May 7, 2008 by Hobbiez

[Tomcat]

How about this question huh? Looks simple but not very simple...can pls help to solve?

 

Thanks.

 

Q: Initial ratio between men and women was 2:3, then the number of men increased by 50 and the number of women decreased by 30. The new ratio becomes 5:6. What is the initial number of men and women?

 

[Kxbc]

M = 300 and W = 450

 

I used the long method (taught in the 70s, 80s) to solve it. Now I think the solution path may not be the same.

Edited May 8, 2008 by Kxbc

[Tomcat]

can show computation or not huh? forgot how to do liao...heeeheee..

[Kxbc]

M/W = 2/3 therefore M = 2/3 W. This is Eq 1.

 

Next equation is (M+50)/(W-30) = 5/6. This is Eq 2.

 

Put Eq 1 into Eq 2 and solve for W. Then use that to solve for M.

[Tomcat]

thanks...

[Mivec9]

let no. of man = x

let no. of woman = y

 

x/y = 2/3

3x = 2y

x = (2/3)y

 

(x+50)/(y-30) = 5/6

6x + 300 = 5y - 150

 

4y + 300 = 5y - 150

y = 450

x = (2/3)(450) = 300

[Mustank]

whoa!!! how come your question all so difficult one

[Tomcat]

nowadays, primary 6 question already so cheam...haiz...luckily passed long ago, otherwise, sure fail...muahaahaaa...

 

thanks for the other method...

[Kxbc]

My method and Xtraun's method are the same. It's just that I too lazy to fully type out the solution.

[Mivec9]

i tot is sec 1....i would say it is a bit difficult for their level. I understand nowadays they learn basic algebra liao.

 

Heard last year psle maths was a killer. Alot of students came out crying Wondering wat they are doing...no wonder change minister already

[Robin]

time to revise my maths....

[Tomcat]

yeah and the method of solving so different...sometime I can solve but not the right method...