How likely is rain tomorrow? Rolling a 6? Probability puts a number on chance. Like a percent, it can be written as a fraction, a decimal or a percent.
Key idea
A probability is a number from $0$ to $1$. $0$ means impossible, $1$ means certain, and $\frac{1}{2}$ means as likely as not. Numbers near $0$ are unlikely; numbers near $1$ are likely.
Why it works
A probability tells what fraction of the time an event happens in the long run. An event that never happens has a fraction of $0$; one that always happens has a fraction of $1$; one that happens half the time has $\frac{1}{2}$. Nothing can happen less than never or more than always, so a probability is never below $0$ or above $1$ (above $100\%$).
Worked examples
Worked example: An impossible event
What is the probability of rolling a 7 on a number cube numbered 1 to 6?
- No face shows 7, so it can never happen.
Answer: $0$, impossible
Worked example: Reading a probability
The chance of rain tomorrow is $0.85$. Describe it.
- $0.85$ is close to $1$.
- Rain is likely, but not certain.
Answer: Likely
Worked example: Not a probability
Which of these cannot be a probability: $0$, $\frac{3}{7}$, $1.2$, $45\%$?
- $1.2$ is more than $1$: more than "certain," which is impossible.
- The others are between $0$ and $1$.
Answer: $1.2$
$\frac{1}{2}$, $0.5$ and $50\%$ are three names for the same probability. Use whichever is easiest to compare.