Mrs.D Math · 7th Grade

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Theoretical Probability
S = {all outcomes}
P = favorable / total
OR / AND / NOT
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🧮 Probability
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🧮 Probability
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📚 Theoretical Probability

The Formula

S = {list all possible outcomes}

P(event) = favorable outcomes ÷ total outcomes

This gives you a fraction. Convert it to a decimal (divide) and a percent (multiply decimal × 100).

1
Count favorable outcomes — how many ways the event can happen.
Rolling an even number on a die: 2, 4, 6 → 3 favorable
2
Count total outcomes — all equally likely possibilities.
A standard die: 1,2,3,4,5,6 → 6 total
3
Write the fraction, then convert:
3/6 → 0.5 → 50%

The Probability Scale

0
Impossible
0.25
Unlikely
0.5
Even Chance
0.75
Likely
1
Certain

Probability is always between 0 and 1 (or 0% and 100%).

Worked Example

A bag has 3 red, 4 blue, 5 green marbles. What is P(blue)?

1
Favorable: 4 blue marbles
2
Total: 3+4+5 = 12 marbles
3
Fraction: 4/12  |  Decimal: 4÷12 = 0.33  |  Percent: 0.33×100 = 33.3%
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QUESTION 1 / 10
FIND THE FRACTION
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