Difficulty Factoring Large Numbers

Hi crisprin117,

The speed with which you can factor a number (especially a big number) is based a bit on your math "skill" and a bit on your pattern-matching ability. Since the Quant section of the GMAT is NOT a "math test" (even though you'll do lots of small calculations), you have to focus more on the pattern-matching and less on the division.

To that end, it helps if you know the quick ways to identify if a number is divisible by 2, 3, 4, 5, 6 or 9. In addition, it's almost always faster to write down a calculation and do the work by hand than to stare off into space and try to do it in your head. Many pacing problems are caused simply because the Test Taker opts not to write things down.

GMAT assassins aren't born, they're made,
Rich

A good general rule on the GMAT is: if you're doing too much computation, you're probably doing something wrong! Most GMAT questions can be solved with minimal computation. For example, if you're given:
(27 x 32 x 36)/(64 x 81)
Don't multiply these together to create big numbers, then try to divide! First, break them down into smaller numbers. We can break all of these into factors of 2 and 3, and then easily cancel:
(3^3 x 2^5 x 2^2 x 3^2)/(2^6 x 3^4)
Then simplify: 3^1 x 2^1 = 6

Always simplify first, and you very rarely end up with big numbers to factor.

But when you do have to factor, here are those divisibility rules that Rich was talking about:
x is divisible by:

2 if x is even

3 if the sum of the digits is divisible by 3. 183 is divisible by 3 because 1 + 8 + 3 = 12

4 if x is divisible by 2 twice, or if the last two digits are divisible by 4. 43,524 is divisible by 4 because 24 is divisible by 4.

5 if x ends in a 0 or 5

6 if x is divisible by 2 and 3

8 if x is divisible by 2 three times

9 if the sum of the digits is divisible by 9

10 if x ends in 0