📚 Probability of Compound Events
The Formula
🔁 P(A OR B) — count outcomes where A or B (or both) occur
⚪ P(A AND B) — count only outcomes where BOTH occur
🔁 Two-Stage: P(A) × P(B) — multiply separate events
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%
