MGMAT Strategy Guide 2 - Algebra Chp 5 Problem Set

[rishjain]

In question no. 10 on page 71, the question asks for the value of X. The solution gives the answer as 'E'.
The question goes like this:
What is the value of X?
1) x=4y-4
2) xy=8

Now each statement individually cannot give the ans. So we take both the statement combine we get X as -8 & 4. Similarly Y we get -1 & 2.

Now I believe the answer should be 'C' as the value of X is both -8 & 4. Whereas, the solution says since you get two different answers that's why it is 'E'

Kindly advice

[Atekihcan]

In GMAT when DS questions asks for the value of a variable or anything, we need to look for an unique answer - a single answer. This may be in terms of value or in terms of either yes or no. If at some point we get two possible answers, then the given information is not sufficient to answer the question.

This is a standard protocol for DS problems.

[srcc25anu]

ST1: x = 4(y-1)
Not Sufficient

St2: xy = 8 or x = 8/y
There can be several possible values of X. If y = 2, x = 4 but if y = 1/2, x = 16
Not Sufficient

Together: 4(y-1) = 8/y or (y-2)(y+1) = 0 implies y is 2 or y = -1
Not sufficient to answer
Ans E

[Brent@GMATPrepNow]

In question no. 10 on page 71, the question asks for the value of X. The solution gives the answer as 'E'.
The question goes like this:
What is the value of X?
1) x=4y-4
2) xy=8

Now each statement individually cannot give the ans. So we take both the statement combine we get X as -8 & 4. Similarly Y we get -1 & 2.

Now I believe the answer should be 'C' as the value of X is both -8 & 4. Whereas, the solution says since you get two different answers that's why it is 'E'

Kindly advice

I just wanted to point out some terminology that can get in the way of these kinds of questions.
Given the quadratic equation that results in this question, we can conclude that x must equal either -8 OR 4

Some students confuse with the fact that the possible values for x are -8 and 4. This does not mean we can say that x equals -8 and 4. x cannot simultaneously equal 2 different values.


Cheers,
Brent

[Blue_Skies]

In question no. 10 on page 71, the question asks for the value of X. The solution gives the answer as 'E'.
The question goes like this:
What is the value of X?
1) x=4y-4
2) xy=8

Now each statement individually cannot give the ans. So we take both the statement combine we get X as -8 & 4. Similarly Y we get -1 & 2.

Now I believe the answer should be 'C' as the value of X is both -8 & 4. Whereas, the solution says since you get two different answers that's why it is 'E'

Kindly advice

For DS , every question should have a unique answer otherwise we can't say with 100% certainty that the values are good. The value of y is -1 or +2. That gives two values of x.Both are good, however we need more info to get the exact value of x.