Hi,

Can someone help me with this question:

Q: If XÂ²+YÂ² = 29, what is the value of (X-Y)Â²?

(1) XY = 10

(2) X = 5

The solution states that (2) is not sufficient as you need the value of XY to solve the problem and X = 5 does not provide enough information to evaluate XY. However, if you know the value of X and you know that XÂ²+YÂ² = 29, then surely this is sufficient?

What am I missing?

Thanks

Kevin

## Official Guide Question 73

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KevinMoon wrote: If XÂ² + YÂ² = 29, what is the value of (X - Y)Â²?

(1) XY = 10

(2) X = 5

**Target question:**

**What is the value of (X - Y)Â²?**

**Given: XÂ² + YÂ² = 29**

IMPORTANT: If we SCAN the answer choices, we can see that it may be useful to REPHRASE the target question.

If we EXPAND (X-Y)Â², we get XÂ² + 2XY + YÂ²

Since we're told that XÂ² + YÂ² = 29, we can replace XÂ² + YÂ² with 29, to get...

(X - Y)Â² = XÂ² + 2XY + YÂ² =

**2XY + 29**

So, to find the value of (X-Y)Â², we need only find the value of XY.

This means we can rephrase the target question as...

**REPHRASED target question:**

**What is the value of XY?**

**Statement 1: XY = 10**

Perfect!

Since we can answer the REPHRASED target question with certainty, statement 1 is SUFFICIENT

*Aside: If we want to go back to the original target question, we can still see that statement 1 is sufficient. We get: (X - Y)Â² = XÂ² + 2XY + YÂ² = 2(10) + 29 = 49. Done!*

**Statement 2: X = 5**

This does not provide enough information to find the value of XY.

Here's why:

If XÂ² + YÂ² = 29, we can replace X with

**5**to get:

**5**Â² + YÂ² = 29

Simplify to get: 25 + YÂ² = 29, which means YÂ² = 4

Solve to get: Y = 2 OR Y = -2

If Y = 2, then XY = (5)(2) = 10

If Y = -2, then XY = (5)(-2) = -10

Since we cannot answer the REPHRASED target question with certainty, statement 2 is NOT SUFFICIENT

Answer = A

*Aside: We have a free video with tips on rephrasing the target question: https://www.gmatprepnow.com/module/gmat- ... cy?id=1100*

Cheers,

Brent

Last edited by [email protected] on Thu Dec 03, 2015 10:06 pm, edited 1 time in total.

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Brent has properly explained this DS questions, so I won't rehash that here.

What I can do is offer some advice about being thorough. The GMAT is a test of patterns (in all sections of the Test), so you have to learn and remember that patterns that will show up AND be prepared to have your knowledge/ability tested in a variety of ways.

Most people know that X^2 = 16 has two solutions (4 and -4), but can you see that EVERY TIME it appears. The GMAT tests certain concepts repeatedly as a way to gauge your ability and assign you a score that you deserve.

ANY time you see a squared-term, you should think "there's probably more than one answer" and then go about the necessary steps to PROVE the possibilities. This is one of the ways that the GMAT can determine your thoroughness.

GMAT assassins aren't born, they're made,

Rich

Pardon me if my question is very silly,[email protected] wrote:KevinMoon wrote: If XÂ² + YÂ² = 29, what is the value of (X - Y)Â²?

(1) XY = 10

(2) X = 5Target question:What is the value of (X - Y)Â²?

Given: XÂ² + YÂ² = 29

IMPORTANT: If we SCAN the answer choices, we can see that it may be useful to REPHRASE the target question.

If we EXPAND (X-Y)Â², we get XÂ² + 2XY + YÂ²

Since we're told that XÂ² + YÂ² = 29, we can replace XÂ² + YÂ² with 29, to get...

(X - Y)Â² = XÂ² + 2XY + YÂ² =2XY + 29

So, to find the value of (X-Y)Â², we need only find the value of XY.

This means we can rephrase the target question as...

REPHRASED target question:What is the value of XY?

Statement 1: XY = 10

Perfect!

Since we can answer the REPHRASED target question with certainty, statement 1 is SUFFICIENT

Aside: If we want to go back to the original target question, we can still see that statement 1 is sufficient. We get: (X - Y)Â² = XÂ² + 2XY + YÂ² = 2(10) + 29 = 49. Done!

Statement 2: X = 5

This does not provide enough information to find the value of XY.

Here's why:

If XÂ² + YÂ² = 29, we can replace X with5to get:5Â² + YÂ² = 29

Simplify to get: 25 + YÂ² = 29, which means YÂ² = 4

Solve to get: Y = 4 OR Y = -4

If Y = 4, then XY = (5)(4) = 20

If Y = -4, then XY = (5)(-4) = -20

Since we cannot answer the REPHRASED target question with certainty, statement 2 is NOT SUFFICIENT

Answer = A

Aside: We have a free video with tips on rephrasing the target question: https://www.gmatprepnow.com/module/gmat- ... cy?id=1100

Cheers,

Brent

i did not understand why should we

Solve for: Y = 4 OR Y = -4

instead of y=2 or y=-2

### GMAT/MBA Expert

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**Posts:**15789**Joined:**08 Dec 2008**Location:**Vancouver, BC**Thanked**: 5254 times**Followed by:**1267 members**GMAT Score:**770

You're absolutely correct. It should be y=2 or y=-2rdds wrote: Pardon me if my question is very silly,

i did not understand why should we

Solve for: Y = 4 OR Y = -4

instead of y=2 or y=-2

I edited my response accordingly.

Cheers and thanks,

Brent

### GMAT/MBA Expert

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- GMAT Instructor
**Posts:**6362**Joined:**25 Apr 2015**Location:**Los Angeles, CA**Thanked**: 43 times**Followed by:**26 members

KevinMoon wrote: ↑Sat Mar 01, 2014 12:52 pmHi,

Can someone help me with this question:

Q: If XÂ²+YÂ² = 29, what is the value of (X-Y)Â²?

(1) XY = 10

(2) X = 5

The solution states that (2) is not sufficient as you need the value of XY to solve the problem and X = 5 does not provide enough information to evaluate XY. However, if you know the value of X and you know that XÂ²+YÂ² = 29, then surely this is sufficient?

What am I missing?

Thanks

Kevin

**Solution:**

We need to determine the total of games that Team A won given that they won 50% of their first 20 games and all the remaining games. Notice that they won 10 of their first 20 games.

**Statement One Alone:**

Since Team A played 25 games altogether, they won the last 5 games. Adding to this the 10 games they won in their first 20 games, they won 15 games altogether. Statement one alone is sufficient.

**Statement Two Alone:**

If we let x be the number of games remaining (after the first 20 games), we can create the equation:

10 + x = 0.6(20 + x)

10 + x = 12 + 0.6x

0.4x = 2

x = 2/0.4 = 20/4 = 5

Since they won all their remaining games, they won the last 5 games. Adding to thisPlus the 10 games they won in their first 20 games, they won 15 games altogether. Statement two alone is sufficient.

**Answer: D**

**Scott Woodbury-Stewart**

Founder and CEO

[email protected]

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