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

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.

Hundreds digit : The hundreds digit is the third digit left of the decimal point. It counts multiples of hundred.

Ones digit : The first digit left of the decimal point (or the digit furthest to the right if there is no decimal point). It counts multiples of 1.

Tens digit : The tens digit is the second digit left of the decimal point. For example 31.2 and 32 both have tens digits equal to 3.

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.

Average : The average or arithmetic mean of numbers $x_1, x_2,\ldots, x_N$ is defined as $\frac{1}{N}(x_1+x_2+\ldots+x_N).$ For example the average of 1 and 2 is $\frac{1}{2}(1+2)=1.5$ and the average of 1, 4 and 10 is $\frac{1}{3}(1+4+10)=5.$

Division : Division is the mathematical operation that divides objects equally into groups. More generally $x\div y$ is defined as the number that if multiplied by $y$ equals $x.$

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

Sum : A sum is the result of an addition.