Seventh Grade · Math · week 32

Probability of simple events

Compute the probability of a simple event as the number of favorable outcomes divided by the total number of equally likely outcomes.

Probability measures how likely an event is, as a number from 0 (impossible) to 1 (certain). For equally likely outcomes, probability of an event = (number of favorable outcomes) / (total number of outcomes). It can be written as a fraction, a decimal, or a percent.

The complement of an event is everything that is NOT that event, and it always has probability 1 minus the event's probability, since together an event and its complement must cover every possible outcome.

📋 For the teacher
Teach it (10–15 min):
  • Say the formula out loud every time: "favorable outcomes over total outcomes."
  • Practice writing the same probability all three ways -- fraction, decimal, and percent -- so switching between them feels automatic.
  • Use the complement rule (1 minus the probability) as a shortcut whenever "NOT" appears in a question.
  • Remind them that a probability must always land between 0 and 1 -- if an answer is negative or bigger than 1, something went wrong.
Talk about it:
  • Why must every probability be a number between 0 and 1?
  • How does the complement rule save you work on a "NOT" question?
  • What would a probability of exactly 0 or exactly 1 mean about an event?

Mastery looks like: They correctly compute the probability of a simple event and its complement, expressing it in fraction, decimal, or percent form.

Common stumbles: Dividing total outcomes by favorable outcomes instead of the reverse; forgetting that a probability can never be negative or greater than 1.

See it visually

Interactive, spoken-aloud tools that show this idea in motion.

DivisionShare fairly and build equal groups.try →
Homework for this lessonType it, write it with a stylus, or print it for pencil & paper.open →
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