Probability is the measure of how likely an event is to happen. It tells us the chance of something occurring, from impossible to certain.
Probability is used in games, weather forecasts, sports, science, and everyday decision-making.
Basic Idea of Probability
Probability compares:
-
Favorable outcomes (what you want to happen)
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Total possible outcomes (all things that could happen)
Formula
Probability = {Number of favorable outcomes} / {Total number of possible outcomes}
Probability Scale
| Probability | Meaning |
|---|---|
| 0 | Impossible |
| 0.5 | Equally likely |
| 1 | Certain |
Probability is usually written as:
-
a fraction ( 1/4 )
-
a decimal (0.25)
-
a percent (25%)
Example 1: Coin Toss
A fair coin has:
-
1 head
-
1 tail
Probability of getting heads:
1/2
Example 2: Rolling a Die
A standard die has numbers 1 through 6.
Probability of rolling a 3:
1/6
Probability of rolling an even number (2, 4, 6):
3/6 = 1/2
Types of Probability
1. Theoretical Probability
Based on what should happen.
Example:
-
Probability of heads on a fair coin = 1/2
2. Experimental Probability
Based on what actually happens during an experiment.
Example:
-
Heads occurred 7 times out of 10 flips:
7/10
Important Probability Rules
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Probability is never less than 0 and never greater than 1
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The sum of probabilities of all possible outcomes equals 1
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If one event becomes more likely, another becomes less likely