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