Basic Probability Flashcards

All 5 cards in this deck

What is the probability of an event when all outcomes are equally likely?

$P(\text{event}) = \dfrac{\text{number of favourable outcomes}}{\text{total number of outcomes}}$

e.g. one red ball out of 5 balls gives $\frac{1}{5}$

What are the smallest and largest values a probability can take, and what do they mean?

$0$ to $1$ inclusive: $0$ means impossible, $1$ means certain.

$0.5$ means an even chance.

How do you find the probability that an event does not happen?

e.g. $P(\text{rain}) = 0.3$

$P(\text{not }A) = 1 - P(A)$, so $1 - 0.3 = 0.7$

What are the steps to find a missing probability in a table of mutually exclusive, exhaustive outcomes?

e.g. $P(\text{red}) = 0.2$, $P(\text{blue}) = 0.5$, $P(\text{green}) = ?$

1. Add the known probabilities: $0.2 + 0.5 = 0.7$

2. Subtract from $1$: $1 - 0.7 = 0.3$

3. $P(\text{green}) = 0.3$

What are the steps to find a probability when the numbers of items are given as multiplicative relationships?

e.g. a bag holds only blue and yellow cubes, with twice as many blue as yellow — find $P(\text{yellow})$

1. Let yellow $= y$, so blue $= 2y$

2. Total $= y + 2y = 3y$

3. $P(\text{yellow}) = \dfrac{y}{3y} = \dfrac{1}{3}$