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by Mathsbuddy » Mon Dec 02, 2013 3:30 am
From x/y = - 10 we get y = -x/10

"percentage of x is x - 10y" = (x - 10y)/x * 100%

Substitute y = -x/10 to get:

(x - 10(-x/10)/x * 100% = (x + x)/x * 100% = 200%

Answer B
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by GMATGuruNY » Mon Dec 02, 2013 4:30 am
The GMAT would specify that x is positive:
amarzouk wrote:If x/y = - 10 and x>0, what percentage of x is x - 10y ?

(A) 10% (B) 200% (C) 50% (D) 100% (E) Cannot be determined
x = -10y.

An alternate approach is to plug in values.
Since one of the answer choices is cannot be determined, test two cases to confirm that each case yields the same percentage.

Case 1: y=-1, x=10
Here, x - 10y = 10 - (10)(-1) = 20.
In this case, (x-10y)/x * 100 = 20/10 * 100 = 200%.

Case 2: y=-2, x=20
Here, x - 10y = 20 - (10)(-2) = 40.
In this case, (x-10y)/x * 100 = 40/20 * 100 = 200%.

In each case, the same percentage is yielded: 200%.

The correct answer is B.
Last edited by GMATGuruNY on Mon Dec 02, 2013 5:18 am, edited 1 time in total.
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by theCodeToGMAT » Mon Dec 02, 2013 4:34 am
x/y = -10

Let x = -10 & y = 1

To find: (x-10y)/x * 100

= (-10 - 10(1))/ -10 * 100

= (-20/-10) * 100

= 200%
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by Mathsbuddy » Mon Dec 02, 2013 5:40 am
GMATGuruNY wrote:The GMAT would specify that x is positive:
amarzouk wrote:If x/y = - 10 and x>0, what percentage of x is x - 10y ?

(A) 10% (B) 200% (C) 50% (D) 100% (E) Cannot be determined
x = -10y.

An alternate approach is to plug in values.
Since one of the answer choices is cannot be determined, test two cases to confirm that each case yields the same percentage.

Case 1: y=-1, x=10
Here, x - 10y = 10 - (10)(-1) = 20.
In this case, (x-10y)/x * 100 = 20/10 * 100 = 200%.

Case 2: y=-2, x=20
Here, x - 10y = 20 - (10)(-2) = 40.
In this case, (x-10y)/x * 100 = 40/20 * 100 = 200%.

In each case, the same percentage is yielded: 200%.

The correct answer is B.
Although it may specify that x is positive,it would not matter even if it were negative as -x/-x = +1 anyway.
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