Skill: Find the probability of a simple event

A simple event is one outcome or group of outcomes you are interested in, such as picking a red marble.

To find its probability:

$$\text{Probability}=\frac{\text{number of favorable outcomes}}{\text{total number of equally likely outcomes}}$$

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

1. Count all possible outcomes.
   There are $(3+2=5)$ marbles in the bag, so the total number of outcomes is $5$.

2. Count the favorable outcomes.
   The favorable outcomes are the $3$ red marbles.

3. Write the probability as a fraction.

$$P(\text{red})=\frac{3}{5}$$

4. Convert if needed.

$$\frac{3}{5}=0.6=60\%$$

So, the probability of choosing a red marble is $\boxed{\frac{3}{5}}$, or $(0.6)$, or $(60\%)$. A probability is always between $0$ and $1$: $0$ means impossible, and $1$ means certain.