abhi47 wrote:

Which of the following numbers is not prime? (Hint: avoid actually computing these numbers.)

A. 6!-1

B. 6!+21

C. 6!+41

D. 7!-1

E. 7!+11

Could please someone expalin the logic behind this ? Even though I picked the right answer while solving I am not pretty clear about the underlying concept?

Thanks,

Abhi

A prime number has only two factors - 1 and itself.

Without calculating, we cannot say whether 6!-1 or 6!+41 will be prime.

But, I can say that 6!+21 will not be prime. The reason is that 6!+21 = 3(1*2*4*5*6 + 7)

(taking 3 common). This means that whatever, the value of 6!+21, it can be written as the product of two numbers: 3 and something else. Hence, this number, 6!+21, definitely has 3 as a factor and hence it cannot be prime.

Since a PS question can have only one correct answer, we don't have to worry about the other options. We can say with certainty that they must be prime.

_________________

Karishma

Veritas Prep | GMAT Instructor

My Blog

Save $100 on Veritas Prep GMAT Courses And Admissions Consulting

Enroll now. Pay later. Take advantage of Veritas Prep's flexible payment plan options.

Veritas Prep Reviews