Basic Rules to Remember


Adding Two Numbers with the Same Sign

Add the two numbers together and keep the sign


Adding Two Numbers with Different Signs

Subtract the two numbers and keep the sign of the larger number.



Multiplying and Dividing Real Numbers

The product or quotient of two numbers with the same sign is a positive number.

2(3) = 6                       -2(-3) = 6


The product or quotient of two numbers with different signs is a negative number.

2(-3) = -6                    -6 / 2 = -3


Do you have a series of multiplication to do with a number of negative and positive signs? Well, count the number of negative signs and if you get:

An even number, your answer will be positive.

An odd number, your answer will be negative.


Zero as a Divisor or Dividend

1.      Any number that is divided by 0 is undefined.

2.      Zero divided by any number is equal to 0.


Combining Like Terms (applies to addition and subtraction)

Remember that apples (x) can only be combined with apples (x), oranges (y) with oranges (y). You cannot combine apples with oranges.

2x + x + 2y + y = 3x + 3y


We can multiply or divide apples and oranges.  

2x(3y) = 6xy               


Percents and Decimals

Always change a percent to a decimal by dropping the percent symbol and moving the decimal two places to the left before multiplying or dividing.

87.5% = .875


If you are given an answer in decimal form and need to know what percent it is, move the decimal two places to the right and add the percent symbol.

.658 = 65.8%