For Parents/Math/What Is Probability?

What Is Probability?

1 min read5th–7th
Data & Probability

Probability measures how likely something is to happen. It is expressed as a number between 0 and 1 (or 0% and 100%).

  • 0 (0%) = impossible (rolling a 7 on a standard die)
  • 0.5 (50%) = equally likely or unlikely (flipping heads)
  • 1 (100%) = certain (the sun will rise tomorrow)

The formula

For events with equally likely outcomes:

Probability = favorable outcomes ÷ total outcomes

Rolling a 3 on a die: 1 favorable ÷ 6 total = 1/6 ≈ 16.7%

Drawing a heart from a deck: 13 hearts ÷ 52 cards = 13/52 = 1/4 = 25%

Key ideas

All probabilities are between 0 and 1. If you calculate something outside this range, there is an error.

The probabilities of all possible outcomes add up to 1. For a die: P(1) + P(2) + P(3) + P(4) + P(5) + P(6) = 1/6 × 6 = 1.

Independent events: One outcome does not affect the next. Flipping heads does not make tails more likely on the next flip.

Expressing probability

The same probability can be written three ways:

  • Fraction: 1/4
  • Decimal: 0.25
  • Percent: 25%

This connects to the fraction-decimal-percent relationship.

Related concepts

Learning path

After this

How to Teach Probability

5th–7th

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