Paper 2 · Probability
⏱ Tested in 1 exam question since 2019
Lesson 7 of 9
Probability 7 – Twin Rule
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- Use the twin rule nCr = nC(n−r) — the two bottom numbers add to the top
- Solve equations like (7 3) = (7 x) or (30 x) = (30 2x) for x
- Link the rule to Pascal's-triangle symmetry: choosing r in = choosing n−r out
Most recent exam questions
- 2019 Paper 2 Q1(a)(i) — probability without replacement (two-stage); when order doesn't matter but the events ar…
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