Skill: Express simple probability as a fraction

When all outcomes are equally likely, write probability as a fraction:

$$\text{Probability}=\frac{\text{number of outcomes that fit}}{\text{total number of possible outcomes}}$$

• The numerator (top number) counts the outcomes you want.
• The denominator (bottom number) counts all possible outcomes.

Example: A bag has 5 marbles: 2 red marbles and 3 blue marbles. What is the probability of choosing a red marble?

1. Count the outcomes that fit: There are 2 red marbles, so the numerator is $2$.
2. Count all possible outcomes: There are 5 marbles altogether, so the denominator is $5$.
3. Write the probability:

$$P(\text{red})=\frac{2}{5}$$

The probability of choosing a red marble is $\frac{2}{5}$. Remember to use the total number of possible outcomes for the denominator, not just the number of red marbles.