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Is 20 the average?

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Is 20 the average? [#permalink] New post 15 Jul 2010, 12:45
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A
B
C
D
E

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Question Stats:

33% (01:47) correct 67% (01:49) wrong based on 7 sessions
Hi,
came across this just now and i believe the OA is wrong, please tell me what you think:

Is 20 the average (arithmetic mean) of u, v, w, and 20?

(1) u + v + w = 60

(2) u - v + w = 20, u + v - w =16

[Reveal] Spoiler:
OA was A...

I believe it should be D - since adding the two equations provided in ST2 cancels out v + w, so we can solve for u, leaving us with 2 equations with 2 unknowns...
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Re: Is 20 the average? [#permalink] New post 15 Jul 2010, 13:12
Expert's post
AndreG wrote:
Hi,
came across this just now and i believe the OA is wrong, please tell me what you think:

Is 20 the average (arithmetic mean) of u, v, w, and 20?

(1) u + v + w = 60

(2) u - v + w = 20, u + v - w =16

[Reveal] Spoiler:
OA was A...

I believe it should be D - since adding the two equations provided in ST2 cancels out v + w, so we can solve for u, leaving us with 2 equations with 2 unknowns...


Have you tried to solve these 2 equations?

When you add u-v+w=20 and u+v-w=16 you'll get u=18 and if you substitute the value of u in these two equations you'll get: for 1. 18-v+w=20 --> w-v=2 and for 2. 18+v-w=16 --> w-v=2, so two identical equations, thus you won't be able to solve for w, v or for w+v.

Statement to be sufficient you need n distinct linear equations for n variables.
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Re: Is 20 the average? [#permalink] New post 19 Jul 2010, 08:10
you can;t solve from B.
u'll get the same equation.
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Re: Is 20 the average? [#permalink] New post 20 Jul 2010, 11:53
AndreG wrote:

[Reveal] Spoiler:
OA was A...

I believe it should be D - since adding the two equations provided in ST2 cancels out v + w, so we can solve for u, leaving us with 2 equations with 2 unknowns...


Using the information in 1) it is possible to solve for v and w in 2). However, in order for D to be correct, we would need to be able to solve for v and w using only the information in 2). Since 1) is sufficient and 2) is insufficient alone, the answer is A.
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Re: Is 20 the average? [#permalink] New post 20 Jul 2010, 12:14
I think as people have pointed out already, you need to make sure you actually know that the equation is solvable. Don't just think that since it simplifies to two equations with two variables you'll be able to solve it. It needs to be two distinct equations.
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Re: Is 20 the average? [#permalink] New post 21 Jul 2010, 12:55
We have to answer Is \frac{U+V+W+20}{4}=20? Or U+V+W+20 is 80

Option 1 gives sum total of all.
Option 2 alone doesn't

So answer is A
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Re: Is 20 the average?   [#permalink] 21 Jul 2010, 12:55
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