ACT Preparation / Unit 15 ACT 1430 short lesson, about 20 seconds
Permutations and Combinations
Captions available
The idea
A permutation counts ordered arrangements: nPr = n!/(n − r)!. A combination counts unordered selections: nCr = n!/(r!(n − r)!).
In practice
Choosing a president and vice president from 10: 10 × 9 = 90. Choosing a 2-person committee: 90/2 = 45.
Where people slip
Ask whether swapping two chosen items makes a new outcome. If it does, order matters.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
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