Session 4: Probability - Module B: Basic Probability

1. Discussion: What is Probability?

What is probability?
How can we turn chance into a number?
How to calculate probability.

Take a coin, and flip it.
What did it land on? Heads or tails?
Whats the chance of that?

Even though the outcomes of flipping coins, rolling dice, drawing cards, and picking things out of a bag are random, we can calculate the likelihood of an event.
As for the coin? It has a probability of 1/2 of being heads, and a 1/2 probability of being tails, meaning that half the time it will land on heads and the rest of the time it will land on tails.

3. Calculating Probability

While this seems obvious, what about rolling a die?
Whats the probability of rolling a three?

To figure out the probability we need to break the problem down.

Probability of an event = number of ways that event can happen
all possible outcomes

For our die, there is only one way to roll a 3 (your die lands with 3 face up), and there are 6 possible outcomes (the die could land on any face from 1 to 6).
So the probability of rolling a three is 1/6.

Now lets go bigger. Calculate the probability for the following events:

• Rolling an even number on a die
• Drawing a king from a deck of cards
• Drawing a red jack from a deck of cards
• Bonus: Rolling a 20 on a 6 sided die

4. Putting Probabilities Together

Sometimes we want to know about probabilities of multiple things.
Like: what's the probability of rolling a 1 OR a 6?
Doing this is simple: we add the probability of rolling a 1, which is 1/6, to the probability of rolling a 6, which is also 1/6, to get 2/6 or reduced to 1/3.

Try this with the following:

• Drawing a king or drawing an ace
• Rolling a number divisible by three or a number divisible by 5
• Drawing a diamond or drawing a black card
• Bonus: Rolling a one or a five, or a six