Leak in the story...

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zagcollins: Master | Next Rank: 500 Posts; Posts: 167; Joined: Wed Jun 11, 2008 10:58 pm; Thanked: 2 times; Followed by:1 members

Leak in the story...

by zagcollins » Mon Jul 21, 2008 8:19 pm

If water is leaking from a certain tank at a constant rate of 1,200 milliliters per hour, how many seconds does it take for 1 milliliter of water to leak from the tank?

A.1/3
B.1/2
C.2
D.3
E.20

I know that this is a simple prob but these prove to be troublesome during the test...whats the best way to tackle such a problem?

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Source: Beat The GMAT — Problem Solving | Mon Jul 21, 2008 8:19 pm

pepeprepa: Legendary Member; Posts: 661; Joined: Tue Jul 08, 2008 12:58 pm; Location: France; Thanked: 48 times

by pepeprepa » Mon Jul 21, 2008 9:43 pm

In many question you need to use what people call "croo-multiplying".

Here is what we know
1,200milimeters ------>1h
In the question, they ask us an answer in seconds, so let's convert our figures.
1,200milimeters ------> 3600seconds
1 milimeter ------> X seconds

Now you can cross multiply the above datas:
3600*1 = 1,200*X
3600/1200=X
X=1/3

If the question would have been, number of milimeters which leak in 45 seconds, you write that:
1,200milimeters ------> 3600seconds
X milimeter ------> 45 seconds
And you cross-multiply in the same way to find X

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zagcollins: Master | Next Rank: 500 Posts; Posts: 167; Joined: Wed Jun 11, 2008 10:58 pm; Thanked: 2 times; Followed by:1 members

by zagcollins » Mon Jul 21, 2008 10:00 pm

hi pepe,

isnt the answer 3 and not 1/3??

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egybs: Master | Next Rank: 500 Posts; Posts: 178; Joined: Wed May 14, 2008 3:51 pm; Thanked: 16 times; Followed by:1 members; GMAT Score:99%+

by egybs » Mon Jul 21, 2008 11:34 pm

we know that it leaks 1200 ml/h
or 1200ml/h / 60 min/h = 20 ml/min
or 20 ml/min /60 sec/min = 1/3 ml/sec

1 ml / 1/3 ml/sec = 3 seconds

No need for cross multiplication... just some division and unit checking.

Also note that you can skip a couple steps:

we know it leaks at 1200ml /3600 seconds
We want to find the how long it takes to drain 1ml... which is really just asking us to find x in:
1 ml / x seconds
So really, all you need to do in this question is divide 3600 by 1200 and get 3.