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What is the sum of the different positive prime factors

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by independent » Fri Apr 13, 2012 12:05 pm
What is the sum of the different positive prime factors of 550

A: 10
B: 11
C: 15
D: 16
E: 18




This question is from the GMATprep, I used the following logic,

550 = 2x225 = 2x15x15 = 2 x 3 x 5 x 3 x 5

Since the question asks for the sum of the DIFFERENT positive prime factors I thought that I should add 2+3+5= [spoiler]C: 10[/spoiler]

But the right question is [spoiler]E: 18[/spoiler]

Now I'm confused, what is meant by different prime factors? Doesn't this mean factors that are not equal, or simply said, factors that do not repeat?
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by GMATGuruNY » Fri Apr 13, 2012 12:12 pm
independent wrote:What is the sum of the different positive prime factors of 550

A: 10
B: 11
C: 15
D: 16
E: 18




This question is from the GMATprep, I used the following logic,

550 = 2x225 = 2x15x15 = 2 x 3 x 5 x 3 x 5

Since the question asks for the sum of the DIFFERENT positive prime factors I thought that I should add 2+3+5= [spoiler]C: 10[/spoiler]

But the right question is [spoiler]E: 18[/spoiler]

Now I'm confused, what is meant by different prime factors? Doesn't this mean factors that are not equal, or simply said, factors that do not repeat?
The portion in red is incorrect.
550 = 2 * 275.

I would factor as follows:
550 = 11*5*10 = 11*5*2*5.

Sum of the different prime factors = 2+5+11 = 18.

The correct answer is E.
Last edited by GMATGuruNY on Fri Apr 13, 2012 12:15 pm, edited 1 time in total.
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by neelgandham » Fri Apr 13, 2012 12:12 pm
Hey independent,

Your approach is perfect but factorisation isn't

550 = 275*2 and not 225*2(=450)
So 550 = 5*11*10 = 5*11*5*2.
Sum of different positive prime factors = 11+5+2 = 18
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by independent » Fri Apr 13, 2012 12:19 pm
Well I'm ashamed now, I focused only on the ''different'' word that I didn't check out my math.

Thank you for the quick response guys!
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by neelgandham » Fri Apr 13, 2012 12:23 pm
Don't worry independent, it happens(it did to me :)). Anyway, good luck with your prep
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