Welcome to Mathnasium’s Math Tricks series. Today we are calculating *basic probability*.

Probability is the *chance* or *likelihood* that something will happen. To calculate the basic probability of something happening, we make a fraction of the number of ways an event can happen (the “favourable” outcomes) out of the total number of “possible” outcomes. Probability can also be represented as a decimal or percent.

The likelihood that something will happen falls between 0 or 0%, (impossible events), and 1 or 100%, (certain events). All other probabilities fall in between 0 and 1.

Follow the examples below to find the basic probabilities of the events.

### Example 1: A bag contains 3 red, 4 white, and 6 blue marbles. If 1 marble is pulled from the bag at random, what is the probability that the marble is red OR white?

Step 1: Find the total number of possible outcomes.

There is a total of: 3 red marbles + 4 white marbles + 6 blue marbles = 13 marbles.

Step 2: Find the total number of favourable outcomes.

The favorable outcome is pulling a red OR white marble from the bag at random.

The total number of favourable outcomes is: 3 red + 4 white marbles = 7 marbles.

Answer:

## Example 2: If a blue marble was pulled from the same bag and not replaced, what is the probability of drawing another blue marble?

Step 1: Find the total number of possible outcomes.

Since a blue marble was pulled from the same bag, there are now: 13 - 1 = 12 marbles.

Step 2: Find the total number of favourable outcomes.

The favourable outcome is drawing another blue marble.

The total number of favourable outcomes is: 6 - 1 = 5 blue marbles.

Answer:

Now, with this strategy, you are ready to calculate the basic probability of an event happening. Click here for more practice problems, then check your answers here.