Probability
Probability is a numerical measure of how likely an event is to happen. Probability is measured in fractions between 0 and 1. (0 is impossible; 1 is certain.) Sometimes, probability is represented as a percentage -- from 0 percent to 100 percent.

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Mean, Mode, and Median
Mean, median, and mode are averages. Mean is the average of a group of numbers. Median is the middle number in a list of numbers that have been arranged in order. Mode is the number that occurs most frequently in a list of numbers arranged in order.

Ratio and Proportion
A ratio is a comparison of two numbers. A proportion is a statement (or equation) that says two ratios are equal. If one number in a proportion is not known, cross-multiplying can be used to find the unknown number.

Explain Decimals
Decimals, like fractions, are a way of writing numbers that are larger or smaller than whole numbers. Decimals are represented by a symbol called a decimal point.

Adding and Subtracting Fractions
To add or subtract like fractions, add or subtract the numerators, and write the sum or difference over the denominator. To add or subtract unlike fractions, you must first find a common denominator.

Explain Regrouping
Grouping numbers makes them easier to work with. And sometimes, regrouping -- arranging numbers into different groups -- is also necessary.

Explain Rounding and Estimating
Rounding and estimating are ways of simplifying numbers to make them easier to understand or work with.

Explain Place Value
Place value helps us determine the value of numbers. Place value means that the value of a digit is determined by its place in the entire number.