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Education Concept Tracker

A prerequisite knowledge graph of 1590 learning concepts across 8subjects. Track what you've mastered and see exactly what you're ready to learn next — the topics whose prerequisites you've already met. Built on the Marble Open Skill Taxonomy.

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Probabilities Sum to One

Mathematics · Probability · conceptual

Understand that when all possible outcomes of a trial are listed, their probabilities must add up to 1; use this to find the probability of an event NOT happening: P(not A) = 1 − P(A); apply this shortcut to avoid counting all unfavourable outcomes directly

What mastery looks like

  • List all outcomes of spinning a 4-colour spinner and verify their probabilities add up to 1
  • Calculate P(not rolling a 3) as 1 − 1/6 = 5/6 using the complement rule
  • Spot an error in a probability table where the values don't sum to 1 and explain what's wrong

Check your understanding

If the probability of it raining tomorrow is 0.3, can Probabilities Sum to One work out the probability of it NOT raining — and explain why all probabilities in a situation must add up to 1?

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