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DS - GMAT PREP - practise test

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ata
 Post subject: DS - GMAT PREP - practise test
 Post Posted: Mon Dec 01, 2008 8:48 pm 
on a certain tour, ratio of number of women to number of children was 10 to 4. what was the no of men on the tour?

(1) ratio of children to men was 10 to 22
(2) no of women on the tour was less than 30

got it right on a fluke but could someone pls explain. ans both together are sufficient.
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djjamal
 Post subject:
 Post Posted: Wed Dec 03, 2008 5:25 am 
Ratio of women (W) to children (C) is 10:4 or 5:2

(1) this only gives a ratio, and the prompt only provides a ratio. With only ratios, there is no way to determine the actual # of men. INSUFFICIENT.

(2) this provides the actual number of women on the tour, but combined with the prompt provides no info on the # of men. INSUFFICIENT.

(1) && (2) since we know that the ratio of W:C is 5:2, W must be a multiple of 5, and C must be even. Let's try the first number under 30 that's a multiple of 5: 25.
W=25, then C=10, then M=22
Let's try another
W=20, then C=8, then M=not a whole number. From this we can tell that M must be a multiple of 22 and C must be a multiple of 10. We can't reduce C:M to 5:11 because the prompt states that C must be even. W=25, C=10, and M=22 is the only combination that works for W<30. SUFFICIENT.
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Joey Z.
 Post subject: Strict thinking
 Post Posted: Fri Dec 05, 2008 8:38 pm 
W:C:M = 100:40:88, among which the greatest common divisor is 4. Divide through by 4, you get:

W:C:M = 25:10:22.
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RonPurewal
 Post subject: Re: Strict thinking
 Post Posted: Mon Dec 15, 2008 6:30 am 
Offline
ManhattanGMAT Staff


Posts: 7146
Joey Z. wrote:
W:C:M = 100:40:88, among which the greatest common divisor is 4. Divide through by 4, you get:

W:C:M = 25:10:22.


yeah, whoa, those are some awfully big numbers. your statement is of course correct, but there's no good reason to use numbers that big.

takeaway:
when you combine 2 ratios to make a larger ratio, use the LEAST COMMON MULTIPLE of the two numbers you're trying to merge.

in this case, you're trying to merge the numbers for "children". the original numbers for children are 2 (because 10:4 reduces to 5:2) and 10, and therefore have a least common multiple of 10.
all you have to do is multiply 5:2 by 5 to give 25:10, whereupon it combines instantly with the other given ratio.

i find it interesting that these ratios were apparently given in non-reduced form; that's somewhat uncharacteristic of the gmat writers. are you sure that statement (2) wasn't given as 5:11?
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tomslawsky
 Post subject: Re: DS - GMAT PREP - practise test
 Post Posted: Wed Aug 05, 2009 3:09 pm 
Offline
Students


Posts: 138
Ron,
your takeaway is succinct and applicable across a few "different" types of questions, and it's easy once you "get it". Thank you.
Tom
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