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Kaplan's GMAT Math Workbook

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Kaplan's GMAT Math Workbook [#permalink] New post 03 May 2012, 13:46
I have a question regarding Kaplan's GMAT Math Workbook. On page 252 (which is Chapter 5, 'Part Three: Question Type Review' on word problems) they give an explanation of a word problem that, to me at least, seems overly complicated. Here is their version:

"Example: Steve is now five times as old as Craig was 5 years ago. If the sum of Craig's and Steve's ages is 35, in how many years will Steve be twice as old as Craig?"
a) 2
b) 5
c) 10
d) 15
e) 25

Kaplan then goes on to explain how to unpack the question and set up an equation to solve. Here is how they set it up:

Let c = Craig's current age
Let s = Steve's current age

(first part of problem) => s=5(c-5)
(second part of problem) => c+s=35
(this is where they seem to complicate matters to me):
c+s=35
c=35-s
s=5(c-5)
s=5(35-s-5)
s=5(30-s)
s=150-5s
6s=150
s=25
(therefore) c=10
(now they complete the final part of the problem and solve for what the problem asks):
25+x=2(10+x)
25+x=20+2x
(answer) x=5

So, as a non-quant, this is what I would like the quants on the forum to answer for me. Why didn't they just do it this way and eliminate a significant step?

s=Steve's age now
c=Craig's age now
c+s=35
s=5(c-5)
c+5(c-5)=35
c+5c-25=35
6c=60
c=10
(therefore) s=25
25+x=2(10+x)
x=5

You get the same result but you don't have to do the whole {c+s=35; c=35-s; s=5(c-5); s=5(35-s-5)} mumbo-jumbo. (mumbo-jumbo is NOT going to appear on the GMAT for all you non-native speakers)

What gives?
_________________

"How far that little candle throws his beams.
So shines a good deed in a weary world."
- Shakespeare

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Kaplan's GMAT Math Workbook   [#permalink] 03 May 2012, 13:46
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Kaplan's GMAT Math Workbook

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