1. Odd Numbers
Definition: An odd number is an integer that cannot be divided exactly by 2.
General Form:
2n + 1 (where n is any integer)
Examples:
1, 3, 5, 7, 9, 11 ...
Key Properties:
Odd + Odd = Even
Odd - Odd = Even
Odd + Even = Odd
Odd - Even = Odd
Odd × Odd = Odd
Odd × Even = Even
GMAT Tip: If a number ends in 1, 3, 5, 7, or 9, it is odd.
2. Even Numbers
Definition: An even number is an integer that is divisible by 2.
General Form:
2n (where n is any integer)
Examples:
2, 4, 6, 8, 10, 12 ...
Key Properties:
Even + Even = Even
Even - Even = Even
Even × Even = Even
Even × Odd = Even
Even ÷ Even = Could be even or odd depending on value
Even ÷ Odd = Not an integer unless divisible
GMAT Tip: If a number ends in 0, 2, 4, 6, or 8, it is even.
3. Positive Numbers
Definition: Numbers greater than zero.
Examples:
1, 2, 3, 4, 0.5, 10.8
Key Rules:
(+) × (+) = (+)
(+) ÷ (+) = (+)
(+) × (-) = (-)
(+) ÷ (-) = (-)
Note: Zero is neither positive nor negative, but it is even.
4. Negative Numbers
Definition: Numbers less than zero, always written with a minus sign (-).
Examples:
-1, -2, -3, -0.5, -10.8
Key Rules:
(-) × (-) = (+)
(-) ÷ (-) = (+)
(-) × (+) = (-)
(-) ÷ (+) = (-)
GMAT Tip: Negative numbers are always smaller than positive numbers - even if their absolute values are larger.
For example: -100 is smaller than +1.
5. Important Relationships
Addition/Subtraction
Odd + Odd = Even
Odd + Even = Odd
Even + Even = Even
Multiplication
Odd × Odd = Odd
Odd × Even = Even
Even × Even = Even
Positive × Negative = Negative
Negative × Negative = Positive
Division
Even ÷ Even → Could be even or odd
Even ÷ Odd → May not be integer
Odd ÷ Even → Never an integer
6. GMAT Strategy Notes
1. If a question says "x is an integer," think about odd/even and positive/negative.
2. If two consecutive integers appear in GMAT questions - one must be odd, one must be even.
3. Zero is even but neither positive nor negative.
4. Always check signs carefully in multiplication/division questions.




