A quick method to identify a prime no?

Expert replies

mitzwillrockgmat: Master | Next Rank: 500 Posts; Posts: 124; Joined: Wed May 19, 2010 10:20 pm; Thanked: 3 times; GMAT Score:1100

by mitzwillrockgmat » Sat Jun 12, 2010 8:39 am

I always take time especially when i'm simplifying fractions to see if a fraction cannot be simplified further.

for instance when you have 314/576 i can quickly simplify it to 157/288 but after that it takes me "some" time to see that, 157 is prime & that i can't simplify further. I try to divide 157 by every no possible so it takes time!! Don't know if others struggle with this but it wastes time during the gmat.

So I'm wondering if anyone has a QUICK method to determine if a no. is prime?

I was able to find one on my own but if there's a FASTER way out there , please share!

my method:

i.e. to see if 157 is prime

find the closest perfect square to this number...that would be 169. 169 is the square of 13.

now divide 157 by all primes less than 13 i.e by 11, 7, 5, 3, 2

if the no. is not divisible by any of these then the no. is prime.

Anyone got a faster method???

Source: Beat The GMAT — Quantitative Reasoning | Sat Jun 12, 2010 8:39 am

tpr-becky: GMAT Instructor; Posts: 509; Joined: Wed Apr 21, 2010 1:08 pmFollowed by: 85 members; Location: Irvine, CA; Thanked: 199 times; GMAT Score:750

by tpr-becky » Sun Jun 13, 2010 2:06 pm

There is no sure formula to determine a prime number - but what you do need to know is the rules of divisibility for the numbers 2- 11

2 - even
3 - sum of digits is divisible by 3
4 - last two digits taken as a number divisible by 4
5 - ends in 5 or 0
6 - even and sum of digits is divisible by 3
7 - just divide it
8 - last three digits taken as a number divisible by 8 (must also be even)
9 - sum of digits is divisible by 9
10 - ends in a 0
11 - just divide it

normally you can run through these very quickly on any given problem to determine prime.

Becky
Master GMAT Instructor
The Princeton Review
Irvine, CA

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