Guessing in the GMAT
There is nothing wrong with guessing on the GMAT. You aren't there to prove you can solve problems by traditional methods. Your aim is to select as many right answers as possible within the allotted time. Nobody will see your calculations; nobody cares how you found your answer. The only things that matter are whether your answer choice is correct and whether found it in time.
A guessing approach on the GMAT is appropriate when it requires much less time than traditional calculations would take or when you can't solve a problem by picking numbers or backsolving.
In order to guess correctly, you need to eliminate as many wrong choices as you can. Luckily many questions in the GMAT will contain answer choices that are obviously wrong or don't make any sense.
It takes father and son 12 hours to paint a building together. If son painted it by himself, it would take him 48 hours. How long it would take dad to paint the same building?
To make this sample question easy to understand and calculate, we will transform it in a more visible form.
1/d + 1/s = 1/t
Let d represent the number of hours the father would take to do the job.
Let s represent the number of hours the son would take to do the job.
Let t represent the number of hours they would take to do the job together.
Now replace the letters d and t with figures from the problem.
1/d + 1/48 = 1/12
1/d = 1/12 – 1/48
1/d = 4/48 – 1/48
1/d = 3/48
1/d = 1/16
d = 1 : (1/16) = 16
It would therefore take the father 8 hours to paint the building on his own, making (B) the right answer choice.
Now try a shortcut and guess the right answer. It is clear that choice (A) is wrong: 10 hours is less than the 12 hours they would need to paint the building together. As the son, obviously, paints slowly on his own, his contribution to the 12 hour combined result would have been less than his father's was. You may then assume that dad by himself would need just a little more than 12 hours, making (B) your guess choice.
It is clear that in this case we would save a lot of time by using guessing approach instead of traditional methods.
If x – y = 10, which of the following must be true?
I. Both x and y are positive.
II. If x is positive, y must be positive.
III. If x is negative, y must be negative.
A) I only
B) II only
C) III only
D) I and II
E) II and III
If you're going to guess, you need to figure out where to start. You want to eliminate as many answer choices as you can, spending as little time as possible. Statement II is included in three of the five answer choices. So it would logical to start with Choice B.
To test it, use the picking-numbers approach. If you try 3 as x, then y must be –7, making statement II incorrect and eliminating answer choices (B), (D), and (E). That leaves answers (A) and (C), giving you a 50/50 chance.
Now improve those odds. Look at statement III. If x = –3, then y must be –13. It meets the requirements of the statement, making answer choice (C) correct and thereby ruling out answer (A).
Was it a pure guessing? No. Was it a pure arithmetic? Again no. So guessing is not the coin-flipping technique its name suggests. It is an intellectual exercise that eliminates wrong answer choices and uses logic to figure out which answer choice is more promising among those that could not be excluded.