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Grade 9 · IB Mathematics

Probability,
Permutations &
Combinations

20 carefully designed problems — covering core formulas, tricky traps, and IB-style reasoning. Work through each question, then reveal the explanation.

Factorial
\(n! = n \times (n{-}1) \times \cdots \times 1\)
COUNT DOWN multiply all the way to 1
Permutation
\(P(n,r) = \dfrac{n!}{(n-r)!}\)
ORDER MATTERS pick r from n, sequence counts
Combination
\(C(n,r) = \dfrac{n!}{r!(n-r)!}\)
ORDER FREE divide extra by r! to kill duplicates
Probability
\(P(A) = \dfrac{\text{favorable}}{\text{total}}\)
WANT ÷ ALL always simplify the fraction
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Great work on IB Math combinatorics!