Count : To count means to find out the number of objects by adding one for every object.

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,...

Standard algorithm : By standard algorithm we refer to any one of the algorithms that are commonly taught at school to add, subtract, multiply or divide multi-digit numbers such as long division and partial products multiplication.

Subtraction : Subtraction is the mathematical operation that describes decreasing a number by an amount equal to a second number. The mathematical symbol for subtraction is the minus sign $-.$ The term subtraction is also used for a generalization of this basic operation on numbers to functions, vectors and matrices.

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 multiplication : The associative property of multiplication says that the product of three numbers $a,b,c$ does not depend on the order in which the product is calculated: $(a\cdot b)\cdot c=a\cdot (b\cdot c).$

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 $2\cdot 3+4=6+4=10.$