# Basics

An integer is a positive or negative number without a fractional or decimal component: -1, 2, 0.

An even number is an integer evenly divisible by two: -4, 0, 2, 8.

An odd number is an integer that is not evenly divisible by two: -5, 3, 9.

A positive number any number > 0. A Negative number any number < 0.

The sum of tree consecutive integers is three times the value of the middle number. $3 + 4 + 5 = 4 * 3$.

3 less than X” means “X - 3”.

• If A, then B” - implication. Let’s say that this is true.
• If B, then A” - converse. May or may not be true.
• If not A, than not B” - inverse. May or may not be true.
• If not B, then not A” - contrapositive. Always true.

If fraction “$5/10$$5$ is numerator and $10$ is denominator. Numerator increasing more rapidly than the denominator.

Unit rate is how much of something there is per one unit of something else (ex: 1 km/h).

Divisibility rules:

• $1$ - all integers are divisible by 1.
• $2$ - the last digit is even: 0, 2, 4, 6, 8.
• $3$ - the sum of the digits is divisible by 3.
• $4$ - the last two digits are divisible by 4.
• $5$ - the last digit is 0 or 5.
• $6$ - the number is divisible by 2 and 3.
• $7$ - double the last digit. Subtract this value from the remaining digits. This number should be divisible by 7.
• $8$ - the last three digits are divisible by 8.
• $9$ - the sum of the digits is divisible by 9.