Addition : Addition is the mathematical operation that describes increasing a number by an amount equal to a second number. The mathematical symbol for addition is the plus sign $+.$ The term addition is also used for a generalization of this basic operation on numbers to functions, vectors and matrices.

Matrix : A $m\times n$ matrix is a rectangular arrangement of $m\cdot n$ numbers into $m$ rows and $n$ columns. For example $\begin{pmatrix}1&2&3\\4&5&6\\\end{pmatrix}$ is a $3\times 2$ matrix. $A_{ij}$ refers to the element of $A$ in the i-th row and j-th column.

Number : A number $x$ is a mathematical symbol representing a quantity.

Skip count : Skip counting refers to repeatedly adding the same number to a starting number. For example skip counting in twos starting at 3 results in 3, 5, 7, 9... Skip counting in threes starting in 1 results in 1, 4, 7, 10,...

Digit : A digit is a symbol used in the representation of a number.

Divisible : An integer $x$ is called divisible by an integer $y$ if the division $x:y$ has a remainder of zero. For example 4 is divisible by 2 but 5 is not divisible by 2.

Divisible by 4 : An integer $x$ is called divisible by 4 if $x$ is a multiple of 4. An integer is divisible by 4 if and only if the last two digits form a number that is divisible by 4.

Number : A number $x$ is a mathematical symbol representing a quantity.

Associative property : An operation * has the associative property if we have $(a*b)*c=a*(b*c)$ for all $a,b,c.$

Associative property of addition : The associative property of addition says that the sum of three numbers $a,b,c$ does not depend on the order in which the sum is calculated: $(a+b)+c=a+(b+c).$

Parenthesis : Parenthesis or brackets are used to clarify the order of operations in an expression. Within an expression terms in parenthesis are supposed to be calculated first. For example $2\cdot(3+4)=2\cdot 7=14$ but $2\cdot 3+4=6+4=10.$