Hi MayankSancheti09,
Determining the factors of a number is a skill that the GMAT will test you on a couple of times (sometimes with prime numbers, sometimes with other subjects). This skill comes down to a couple of factors:
1) How well do you know your multiples?
2) How quickly can you "do math" when it's required?
For children, the idea of "times tables" is something that they learn fairly early on in school. That knowledge can come in quite handy during the Quant section, so if your math skills are a bit rusty, then you need to put in some time and build them back up. Your ability to add, subtract, multiply and divide 1 and 2-digit numbers must be flawless.
Along with those skills, there are a variety of Number Property shortcuts that can help you to figure out if a number is divisible by another number.
As an example, there's the "rule of 3" - IF the digits of a number add up to a number that is divisible by 3, then the ORIGINAL number is divisible by 3.
For example, the number 27 is divisible by 3 because 2+7 = 9 and 9 is divisible by 3, so 27 is divisible by 3.
Most "prime number" questions on the GMAT don't involve big numbers or complex calculations, so you shouldn't be concerned.
One of your examples is 139. How quickly could you figure out if it was prime or not? I'll give you a nudge: even numbers do NOT divide evenly into odd numbers, so you don't have to think about 2,4,6,8,10,etc. Is 139 divisible by 3? How about 5? You should quickly be able to eliminate most of the possibilities based on logic. For the other numbers, you'll have to physically do some math to prove what's possible.
GMAT assassins aren't born, they're made,
Rich
