GMAT Tip of the Week: Leveraging Answer Choices

GMAT Tip of the WeekIf GMAT tutoring sessions sometimes look like George (or Oscar) Bluth prison meetings from Arrested Development – two people across the table from each other speaking intelligently – the “no touching” recurring theme is embedded in this exchange:

Step one: Student begins to work on problem, places scratch paper directly underneath problem covering answer choices.
Step two: Instructor slaps the note paper away and yells “no touching (the answer choices)”

Why?

Particularly on Problem Solving questions, the answer choices are often the most important assets you have in solving the problem. Some problems require you to plug in answer choices (“backsolve”) in order to solve; other problems embed clues in the answer choice (if there’s a square root of 3, you should be looking for a 30-60-90 triangle somewhere; if all the denominators in the answer choices are either 3 or 5, you should be thinking about divisibility rules). A higher-than-you’d-think percentage of Problem Solving questions reward users for glancing at the answer choices before they start their work, but a higher-than-you’d-think percentage of students never look past the question mark in the problem before they diligently start calculating. Let’s see a few examples to show you how looking at answer choices can drastically increase your efficiency and accuracy:

Which of the following is equal to 124/93?

(A) 6/5
(B) 5/4
(C) 4/3
(D) 3/2
(E) 8/9

If you were to try to factor out the common term between 124 and 93, you’d have a tough time identifying it on its own. 124 = 4(31) and 93 = 3(31), but very few people will quickly see “oh, they’re both divisible by 31″. Instead, you’re much more likely to make that determination by looking at the answer choices. Choices A, B, and D are clearly wrong because the denominator – 93 – is not divisible by 5 and not even, so it cannot factor down to have a denominator of 5, 4, or 2. And choice E should be clearly wrong because in the original, 124/93, the numerator is greater than the denominator, but choice E reverses that. So C is the only plausible choice, and if you test it it gives you a clue as to what to factor out. You’d need to divide the numerator, 124, by 4 (leaving 31) and then test the denominator to make sure it’s also divisible by 31 (and it is, producing that 3).

When you need to reduce a fraction as the last step of a problem, try looking at the answer choices for clues as to which factors to break out – after all, one of the answer choices MUST BE correct, and several should be impossible to begin to factor, thereby lightening your load.

Take a look at another example:

3^8 + 3^7 – 3^6 – 3^5 =

(A) (3^5)(2^4)
(B) (3^6)(2^5)
(C) (3^5)(2^6)
(D) 6^5
(E) none of the above

If you aren’t sure how to even start the problem, look at the answer choices – none of them has addition or subtraction, and most of them involve multiplication. So what’s your next move? Make your math look like the answer choices – you have to factor away that add/subtract to form multiplication (try it and see if you can D-termine the answer).

The takeaway – answer choices are an absolutely integral part of problem solving questions, so make sure to glance at them before you begin your work, and to lean on them if you’re struggling at any point of your calculation. Answer choices are assets!

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