dave13 wrote:

Italiandrummer97 wrote:

Ratio A=1/20

Ratio B= 1/30

Combined ratio= 1/20+1/30=1/12

Now, since we know that tap A was opened before B, we can create the function:

1/20*x + 1/12*(15-x)=1

Substitute the values and you'll find that the correct answer is B.

bonjurno

, isnt tthere a mistake in your equation ?

why didnt you write equation like this

1/20*x + 1/30*(15-x)=1/12

from which source did you learn work and rate problems

Hey

No, I don't think so: the reasoning behind the equation is this:

1)\(1/20x\) Is the ratio of A * time it is activated

2)\(1/12*(15-x)\) Is the ratio after you open tap B, which working together with tap A gives a combined ratio of 1/12, *time they are activated together(i.e. total time - x = 15-x).

3)Equate their sum to 1, which is the cistern we have to fill.

Then solve the equation and the result will be \(15/2\), hence the correct answer is B.

I learned to create models in high school and university, I just applied this knowledge to this work and rate problem

I'm sure there are other ways, but that's the one that came up in my mind at the moment.

If you need any further clarification just quote me and I'll be happy to help

And please, give kudos if you found this solution helpful

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My GMAT history:

1st GMATPrep: 620 (Q46, V29)

2nd GMATPrep: 700(Q46, V40)

3rd GMATPrep: 720(Q49, V38)