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AVERAGE OF X AND Y

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by oquiella » Mon Oct 05, 2015 4:51 pm
IF THE AVERAGE OF X AND Y IS 60 AND THE AVERAGE OF Y AND Z IS 80, WHAT IS THE VALUE OF Z-X?

A. 70
B. 40
C. 20
D. 10
E. IT CANNOT BE DETERMINED FROMTHE INFORMATION GIVEN.
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by Brent@GMATPrepNow » Mon Oct 05, 2015 4:57 pm
oquiella wrote:IF THE AVERAGE OF X AND Y IS 60 AND THE AVERAGE OF Y AND Z IS 80, WHAT IS THE VALUE OF Z-X?

A. 70
B. 40
C. 20
D. 10
E. IT CANNOT BE DETERMINED FROMTHE INFORMATION GIVEN.
THE AVERAGE OF X AND Y IS 60
So, (X + Y)/2 = 60
Multiply both sides by 2 to get: (X + Y) = 120

THE AVERAGE OF Y AND Z IS 80
So, (Y + Z)/2 = 80
Multiply both sides by 2 to get: (Y + Z) = 160

Now SUBTRACT the blue equation from the red equation to get:
(Y + Z) - (X + Y) = 160 - 120
Simplify: Z - X = 40
Answer: B
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by oquiella » Wed Oct 07, 2015 8:09 am
Brent@GMATPrepNow wrote:
oquiella wrote:IF THE AVERAGE OF X AND Y IS 60 AND THE AVERAGE OF Y AND Z IS 80, WHAT IS THE VALUE OF Z-X?

How did you simplify to Z-X. Can you explain?
A. 70
B. 40
C. 20
D. 10
E. IT CANNOT BE DETERMINED FROMTHE INFORMATION GIVEN.
THE AVERAGE OF X AND Y IS 60
So, (X + Y)/2 = 60
Multiply both sides by 2 to get: (X + Y) = 120

THE AVERAGE OF Y AND Z IS 80
So, (Y + Z)/2 = 80
Multiply both sides by 2 to get: (Y + Z) = 160

Now SUBTRACT the blue equation from the red equation to get:
(Y + Z) - (X + Y) = 160 - 120
Simplify: Z - X = 40
Answer: B
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by [email protected] » Wed Oct 07, 2015 9:27 am
Hi oquiella,

You can also answer this question by TESTing VALUES.

We're told that:
(X+Y)/2 = 60
(Y+Z)/2 = 80

So....
X+Y = 120
Y+Z = 160

Let's TEST....
X = 20
Y = 100
Z = 60
Z - X = 60 - 20 = 40

Now, you might be asking yourself "what if I TEST different VALUES for X, Y and Z?" Go ahead and try it...

Let's TEST...
X = 60
Y = 60
Z = 100
Z - X = 100 - 60 = 40

That's the same answer as before, from a completely different set of values. No matter what values you try, as long as they 'fit' the information in the prompt, then the answer will always be the same.

Final Answer: B

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by patco » Wed Oct 07, 2015 1:26 pm
Brent@GMATPrepNow wrote:
oquiella wrote:IF THE AVERAGE OF X AND Y IS 60 AND THE AVERAGE OF Y AND Z IS 80, WHAT IS THE VALUE OF Z-X?

A. 70
B. 40
C. 20
D. 10
E. IT CANNOT BE DETERMINED FROMTHE INFORMATION GIVEN.
THE AVERAGE OF X AND Y IS 60
So, (X + Y)/2 = 60
Multiply both sides by 2 to get: (X + Y) = 120

THE AVERAGE OF Y AND Z IS 80
So, (Y + Z)/2 = 80
Multiply both sides by 2 to get: (Y + Z) = 160

Now SUBTRACT the blue equation from the red equation to get:
(Y + Z) - (X + Y) = 160 - 120
Simplify: Z - X = 40
Answer: B
Nice and extremely fast. Thank you! :)
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