Please clarify

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Arunlal: Junior | Next Rank: 30 Posts; Posts: 15; Joined: Sat Aug 30, 2008 4:36 pm; GMAT Score:530

by Arunlal » Thu Oct 16, 2008 2:03 am

I found this qn in OG. just couldnt understand the last few things...

The Water from an outlet, flowing at a constant rate, can fill a swimming pool in 9 hours. a second outlet can fill the same pool in 5 hours. how long will it take to fill the same pool if both outlet are used together.

together they can fill at the rate of 1/9+1/5= 14/45. ( i could understand this. but couldnt understand the below statements.

" thus when the outlets are used together they fill the pool in 45/14=3.21 hrs"

How is the above derived. i coulodnt understand this. am i missing out on something basic??

-Arun

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Source: Beat The GMAT — Problem Solving | Thu Oct 16, 2008 2:03 am

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lunarpower: GMAT Instructor; Posts: 3380; Joined: Mon Mar 03, 2008 1:20 amFollowed by: 1535 members; Thanked: 2256 times; GMAT Score:800

Re: Please clarify

by lunarpower » Thu Oct 16, 2008 2:20 am

Arunlal wrote:I found this qn in OG. just couldnt understand the last few things...

The Water from an outlet, flowing at a constant rate, can fill a swimming pool in 9 hours. a second outlet can fill the same pool in 5 hours. how long will it take to fill the same pool if both outlet are used together.

together they can fill at the rate of 1/9+1/5= 14/45. ( i could understand this. but couldnt understand the below statements.

" thus when the outlets are used together they fill the pool in 45/14=3.21 hrs"

How is the above derived. i coulodnt understand this. am i missing out on something basic??

yes, it's pretty basic. it's also very important, so make sure that you learn the following somehow, whether by putting it on a flash card or by ... something:

the TIME taken to perform one job is the RECIPROCAL OF THE RATE at which that job is performed.

so, since the combined rate is 14/45, the time taken to perform one combined job is the reciprocal of that number, or 45/14.

if you want to, you could also derive this number by using a good old fashioned RTD equation: (14/45 pool/hr)(x hours) = 1 pool; therefore, x = 45/14 hour. since the right-hand side is 1 job (1 pool, in this case), you'll notice that this scenario will give you the reciprocal in all cases.

Ron has been teaching various standardized tests for 20 years.

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rohangupta83: Legendary Member; Posts: 541; Joined: Thu May 31, 2007 6:44 pmFollowed by: 3 members; Location: UK; Thanked: 21 times; GMAT Score:680

Re: Please clarify

by rohangupta83 » Thu Oct 16, 2008 2:27 am

Arunlal wrote:
together they can fill at the rate of 1/9+1/5= 14/45. ( i could understand this. but couldnt understand the below statements.

the above statement means that 14 parts of the pool (out of 45 parts in total) can be filled in 1 hour.

so in 1 hour 14/45 parts can be filled

in 2 hours 2X14/45 parts = 28/45 parts can be filled

in 3 hours 42/45 parts can be filled

in 3.21 hours 45/45 parts can be filled = 1 pool

or in another way:

14/45 parts are filled in 1 hour

1 part is filled in 1 hour X 45/14

Arunlal wrote: " thus when the outlets are used together they fill the pool in 45/14=3.21 hrs"

Hope this clears things up.

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