Number Properties — Tested Heavily

Even/Odd rules:

Even + Even = Even. Odd + Odd = Even. Even + Odd = Odd.

Even x Anything = Even. Odd x Odd = Odd.

Positive/Negative:

Positive x Positive = Positive. Negative x Negative = Positive.

Positive x Negative = Negative.

Divisibility rules:

Divisible by 2: last digit even

Divisible by 3: digit sum divisible by 3

Divisible by 5: ends in 0 or 5

Divisible by 6: divisible by both 2 and 3

Divisible by 9: digit sum divisible by 9

Prime numbers under 30: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29

2 is the only even prime.

GRE favorite: Questions about remainders, factors, and prime factorization. Know how to break any number into its prime factors.

Reference:

Wikipedia: Number Theory

https://en.wikipedia.org/wiki/Number_theory