Lesson: Common GMAT Data Sufficiency Myths - Part I

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Comment on Common GMAT Data Sufficiency Myths - Part I

Thank you Brent for these tips. Amazing. But I have a question, any indicators when we should calculate and solve for the quadratic equations please!
Best Regards
Fatima-Zahra
gmat-admin's picture

Great question!

In most cases, it's a good idea to solve the quadratic.

For example, let's say that a certain a Data Sufficiency question asks you to determine the number of children in a certain group. In the process of solving the question, you let x = the number of children in a certain group.

Then, from one of the given statements, you're able to create the following equation: x² + 5x - 14 = 0

At this point, you might recognize that the equation has two different solutions. HOWEVER, before concluding that the statement is NOT sufficient, you might want to first solve the quadratic equation.

When you solve x² + 5x - 14 = 0, you get TWO solutions: x = 2 and x = -7.

HOWEVER, in the real world, we cannot have -7 children in a group. So, we can eliminate the possibility that x = -7, and conclude that x MUST equal 2, which means the statement is SUFFICIENT.

Cheers,
Brent

Thank you very much Brent!!

Hi Brent,

x² + 5x - 14 = 0

In above equation we can have two answer either (-2 or 7) or,(-7 or 2) so the student knows x can be 2 or 7.
So can we conclude this equation is insufficient?
gmat-admin's picture

Hi sachindanid,

If the target question were "What is the value of x?", then the statement x² + 5x - 14 = 0 would be insufficient.
Let's examine why this is the case...

Given: x² + 5x - 14 = 0
Factor to get: (x + 7)(x - 2) = 0
So, EITHER x + 7 = 0 OR x - 2 = 0
So, our two possible solutions are: x = -7 and x = 2
As such, the statement is not sufficient.

Does that help?

Cheers,
Brent

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