General Mistake #1: Not reading the problem carefully

Generally under timed conditions you will always have a compelling feeling to rush. This can lead to misreading the sentence or miss some key words changing the whole context of the problem thus making a straight forward problem convoluted.
Worse yet, you may get a numeric entry question and blithely enter in the wrong answer, something you could easily have avoided doing had you read the question carefully.

General Mistake #2: Flubbing the Math

Many math mistakes result from forgetting something so minor as write a negative sign. Other times, simple mathematical errors, like thinking that 16 x 5 = 90 can be very costly. Math is about precision so use your prep time to become an efficient and unerring human calculator.

Specific Mistakes

Below are two common mistakes/oversights, along with problems that test those mistakes. See if you can avoid these common GRE mistakes.

Prime Numbers

2 is the smallest prime number. It is the only even prime. 1 is NOT a prime.

Don’t Forget 0 and 1

Especially in Quantitative Comparison, you always want to make sure to plug in 0 and 1 if the constraints permit doing so. Oftentimes plugging in a 0 or 1 will prove the exception, thus making the answer (D).

Explanation

Remember to plug in ‘0’ and ‘1.’ If you don’t, you will choose the obvious – but actually incorrect – answer (B). However, if x is zero the two columns are equal. Because of this one instance, we cannot say for sure which side is bigger. Thus (D).

Must Be vs. Could Be

There is a subtle, but important difference here. If a question is phrased ‘must be’, then the answer you choose must always hold true for the conditions stated in the problem.

‘Could be’ means that in certain instances, i.e. for certain numbers.

All of this makes a lot more sense when in the context of the problem. So let’s take a look at number 1.

1. c and d are prime numbers. If c - d is an odd prime, then which of the following must be true?

(A) c is even

(B) d is odd

(C) \(c*d\) is odd

(D) d is even

(E) \(c*d - c\) is even

Explanation

First off, don’t let the variables throw you. There is an answer, so there must be some pattern that you have to discern.

If you remember, I mentioned that ‘2’ is the only even prime. Thus the rest are all odds. The question says that c - d is an odd prime. The only way to get an odd number when we subtract two numbers is that one number must be odd and one must be even.

Since ‘2’ is the only even prime we know that ‘2’ must be d. (c cannot equal ‘2’ because c - d would end up being negative number, and primes can’t be negative).

We don’t have to know what exact number c equals. As long as c - d equals an odd prime. c = 5 is perfect. We plug in those values into the question.

Only D works. And we know that d must be even, because d must equal 2, an even number.