Probabilities Sum to One
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
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Probabilities Sum to One means practising the idea at the table with counters or paper models and one clear check. We will demo once, then your child tries.
Materials: 20 counters or beans; Paper and pencil for recording sums. Adult models 7 + 5 with counters, then writes the sum. Child builds three addition problems with counters and records each sum. Done: three correct sums with counters matching the written numbers. Stop after the fresh check; keep optional challenge for another day.
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Start practiceProbabilities Sum to One practice
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PhET Interactive Simulations · Practice link — not the full lesson
Plinko Probability
Before you open · source notes
Plinko Probability supports hands-on exploration for Probabilities Sum to One. Not every lab worksheet.
Free interactive page, no sign-in required for the tested path.
Before you openAsk the learner what they changed in the sim or page and what they observed.
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Source, access and review notes
Public PhET Interactive Simulations page. An anonymous GET on 2026-11-12 returned HTTP 200 with no login or payment prompt for this URL. Opening the link does not save practice or assess mastery. An adult should preview and guide the learner.
PhET Interactive Simulations. “Plinko Probability.” External page; Gakuva does not copy, host, embed, or redistribute materials.
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Check understanding
- 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
“If the probability of it raining tomorrow is 0.3, can your child work out the probability of it NOT raining — and explain why all probabilities in a situation must add up to 1?”
Curriculum record
- Type
- Conceptual
- Subject
- Mathematics
- Domain
- Probability
- Age range
- Ages 10–11
Standards