Rat' Expression problem: how long to fill tank?

[Danielita]

Hi I need help how to solve this problem:

One pipe can fill a tank with water in 20 minutes. Another pipe could fill the same tank in 15 minutes.

If both pipes are running, what is the rate at which water is flowing into the tank (in tanks per minute)?

How long would it take both pipes to fill a tank?

So far I what I did is this:

. . . 1/15 + 1/20 = x
. . .5/20 + 1/20 = 6/20

But I don't think you solve it like that because my teacher did it a different way that I don't remember.

Thanks for your help :lol:

 

[stapel]

So far I what I did is this:

. . . 1/15 + 1/20 = x
. . .5/20 + 1/20 = 6/20

How did you arrive that these equations? What does "x" stand for?

Please reply with your steps and reasoning. Thank you.

Eliz.

 

[skeeter]

pipe #1 can fill the tank in 20 minutes ... it fills at the rate of (1 tank)/(20 minutes)

pipe #2 can fill the tank in 15 minutes ... it fills at the rate of (1 tank)/(15 minutes)

together, they fill the tank at a rate of (1 tank)/(20 minutes) + (1 tank)/(15 minutes)

let t = amount of time to fill the tank together (in minutes) ...

[(1 tank)/(20 minutes) + (1 tank)/(15 minutes)](t minutes) = 1 tank filled

without all the units ...

[(1/20) + (1/15)]t = 1

solve for t