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Qs for Plugging in No.s

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Qs for Plugging in No.s

by Digvijay01 » Wed Oct 30, 2013 10:29 am
Hi.. this is a Qs from Magoosh..
Can you tell.. how can we do the below Qs by plugging in no.s. I already know algebraic method.

The average (arithmetic mean) of y numbers is x. If 30 is added to the set of numbers, then the average will be x - 5. What is the value of y in terms of x ?

a. x/7-5
b. x/6-6
c. x/6-5
d. x/5-7
e. x/5-6
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by GMATGuruNY » Wed Oct 30, 2013 11:20 am
Digvijay01 wrote:Hi.. this is a Qs from Magoosh..
Can you tell.. how can we do the below Qs by plugging in no.s. I already know algebraic method.

The average (arithmetic mean) of y numbers is x. If 30 is added to the set of numbers, then the average will be x - 5. What is the value of y in terms of x ?

a. x/7-5
b. x/6-6
c. x/6-5
d. x/5-7
e. x/5-6
Plug in y=1 -- implying that the original set includes only ONE value -- and solve for x.

Original set:
Sum = (number)(average) = 1*x = x.

New set:
Here, there are 2 values: the original value and 30.
Since the new average = x-5, the sum of the 2 values = (number)(average) = 2(x-5) = 2x-10.

Since the new set includes 30, while the original set does not, the difference between the two sums is 30:
(2x - 10) - x = 30
x=40.

The question stem asks for the value of y=1.
Thus, the correct answer choice must yield a value of 1 when we plug in x=40.
Only D works:
(x/5) - 7 = 40/5 - 7 = 1.

The correct answer is D.
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by Digvijay01 » Wed Oct 30, 2013 12:53 pm
Thanks much...!! That helps a lot... so the take away from me is to keep it simple.. :)
GMATGuruNY wrote:
Digvijay01 wrote:Hi.. this is a Qs from Magoosh..
Can you tell.. how can we do the below Qs by plugging in no.s. I already know algebraic method.

The average (arithmetic mean) of y numbers is x. If 30 is added to the set of numbers, then the average will be x - 5. What is the value of y in terms of x ?

a. x/7-5
b. x/6-6
c. x/6-5
d. x/5-7
e. x/5-6
Plug in y=1 -- implying that the original set includes only ONE value -- and solve for x.

Original set:
Sum = (number)(average) = 1*x = x.

New set:
Here, there are 2 values: the original value and 30.
Since the new average = x-5, the sum of the 2 values = (number)(average) = 2(x-5) = 2x-10.

Since the new set includes 30, while the original set does not, the difference between the two sums is 30:
(2x - 10) - x = 30
x=40.

The question stem asks for the value of y=1.
Thus, the correct answer choice must yield a value of 1 when we plug in x=40.
Only D works:
(x/5) - 7 = 40/5 - 7 = 1.

The correct answer is D.
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