Lesson: Avoiding Common Mistakes - Part I

Comment on Avoiding Common Mistakes - Part I

Thank you. That was really helpful

Just to clarify, for don't overcalculate it says to not calculate the statement just determine if there is sufficient information to answer the question.

By that logic 4.25x-11.75=14.75 should be sufficient? If not, how do you evaluate sufficiency without calculating?
gmat-admin's picture

The target question asks "Does x = 6?"

IF we were to solve the equation, 4.25x - 11.75 = 14.75, we'd get ONE solution. At that point, we'd be able to definitively answer the target question.

Since we COULD answer the target question with certainty, that statement is sufficient.

All we need to know here is that the equation 4.25x - 11.75 = 14.75 will yield exactly one solution. Given this, we don't have to solve the equation for x.

Does that help?

Cheers,
Brent

Hi Brent,

What if it solves for a value other than 6? How is it sufficient. I am having a bit of a hard time understanding this concept.
gmat-admin's picture

For Data Sufficiency questions, if there's only ONE answer to the target question, then the statement is sufficient.

If the target question is "Does x = 6?" then there are only two possible answers: YES or NO.

So, if statement 1 is "2x = 12," then there's only ONE answer to the target question: YES, x IS equal to 6.
Since we can be certain of the answer to the target question, statement 1 is sufficient.

Likewise, if statement 1 is "2x = 10," then there's only ONE answer to the target question: NO, x is NOT equal to 6.
Since we can be certain of the answer to the target question, statement 1 is sufficient.

However, if statement 1 is "x < 8," then there's MORE THAN ONE answer to the target question:
If x < 8, it could be the case that x = 7, which means the answer to the target question is: NO, x is NOT equal to 6.
If x < 8, it could also be the case that x = 6, which means the answer to the target question is: YES, x IS equal to 6.
Since we can't answer the target question with certainty, statement 1 is NOT sufficient.

Does that help?

Yes this helps. Thank you.

Ohhhhhhhhh, I got it now. Basically we have sufficient information to answer yes/no to the target question with certainty, regardless of the outcome for x.
gmat-admin's picture

Exactly!

thanks

Hi Brent, just on overcalculating, what if the equation solving for x in the option doesn't equal 6? for example when we solve for x we get x= 3, so that is not sufficient right?
gmat-admin's picture

If the target question asks "Does x = 6?" and statement 1 says "x + 1 = 4", then statement 1 would still be sufficient, because we can still answer the target question with absolute certainty.

That is, I can provide an definitive answer to the target question: NO, x definitely does NOT equal 6.

You're not alone. Many students compare the answer to the target question with the answer to the sufficiency question. This is covered starting at 3:28 in the above video.

Cheers,
Brent

Great explanation, got that! so when the Q says "is x=6", it means can you answer if either x is or not equal to 6 with certainty. thanks!
gmat-admin's picture

Exactly!

Hi Brent,

This is such an informative video. It would save a lot of time!

Thanks.

I have made this mistake in a few questions too. Thanks for the explanation, it was on point.

Would there ever be a situation in a yes/no data sufficiency question where statement 1 the answer to the target question is yes and statement 2 the answer to the target question is no?
gmat-admin's picture

Great question!
The statements will never contradict each other. So, you'll never see a Data Sufficiency question in which statement 1 yields YES as the answer to the target question, and statement 2 yields NO.
I cover this in a later video: https://www.gmatprepnow.com/module/gmat-data-sufficiency/video/1104

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