• 7 CATs FREE!
If you earn 100 Forum Points

Engage in the Beat The GMAT forums to earn

### GMAT/MBA Expert

GMAT Instructor
Posts: 4430
Joined: 25 Apr 2015
Location: Los Angeles, CA
Thanked: 43 times
Followed by:21 members
by Scott@TargetTestPrep » Fri May 24, 2019 3:12 pm
M7MBA wrote:How many prime numbers exist between 200 and 220?

(A) None
(B) One
(C) Two
(D) Three
(E) Four

The OA is B.
First, we can omit all the even numbers (since they are divisible by 2) and all the odd numbers ending in 5 (since they are divisible by 5). So we are left with 201, 203, 207, 209, 211, 213, 217 and 219. We can omit 201, 207, 213 and 219 also since all of these numbers are divisible by 3 (notice that the sum of their digits is divisible by 3). So we only need to consider 203, 209, 211 and 217.

203/7 = 29 â†’ So 203 is not a prime.

209/7 = 29 R 6, 209/11 = 19 â†’ So 209 is not a prime.

211/7 = 30 R 1, 211/11 = 19 R 2, 211/13 = 16 R 3, 211/17 = 12 R 7 â†’ So 211 is a prime.

217/7 = 31.â†’ So 217 is not a prime.

Therefore, there is only 1 prime number between 200 and 220.