Basic Probability Flashcards

All 10 cards in this deck

What is the range of values a probability can take?

From $0$ to $1$ inclusive.
$0$ = impossible, $1$ = certain.

Which word describes a probability of exactly $\frac{1}{2}$ on the probability scale?

Evens (even chance).

How do you write the probability of an event when all outcomes are equally likely?

e.g. 4 red sweets out of 15 sweets

$P(\text{event}) = \dfrac{\text{number of successful outcomes}}{\text{total number of outcomes}}$
e.g. $\frac{4}{15}$

True or false? A probability may be written as a ratio such as $4:15$.

False.
Probability must be a fraction, decimal or percentage — $4:15$ scores no marks.

What is the formula for the probability of an event $A$ not happening?

$P(\text{not }A) = 1 - P(A)$
e.g. if $P(\text{six}) = 0.3$ then $P(\text{not six}) = 0.7$

What do the probabilities of a set of mutually exclusive outcomes that covers everything (exhaustive) add up to?

$1$.
So a missing probability $= 1 -$ (the others).

What does it mean for two events to be mutually exclusive, and what can you then do with their probabilities?

They cannot both happen at the same time; then $P(A\text{ or }B) = P(A)+P(B)$.

True or false? For a single roll of a dice, 'even number' and 'greater than 3' are mutually exclusive, so you can add their probabilities.

False.
4 and 6 are both, so the events overlap and adding is invalid.

What are the steps to find the total number of items from a given probability?

e.g. $P(\text{blue}) = 0.2$ and there are 12 blue cubes

1. Write probability $=\dfrac{\text{number of that type}}{\text{total}}$: $0.2 = \frac{12}{\text{total}}$
2. Divide the number by the probability: $12 \div 0.2$
3. Total $= 60$ cubes

What does 'taken at random' mean in a probability question?

Every item is equally likely to be chosen — no item is favoured.