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Suppose we want to choose 5 objects, without replacement, from 15 distinct objects.
(a) How many ways can this be done, if the order of the choices does not matter?
(b) How many ways can this be done, if the order of the choices matters?

Suppose we want to choose 55 objects, without replacement, from 1515 distinct objects.\newline(a) How many ways can this be done, if the order of the choices does not matter?\newline(b) How many ways can this be done, if the order of the choices matters?

Full solution

Q. Suppose we want to choose 55 objects, without replacement, from 1515 distinct objects.\newline(a) How many ways can this be done, if the order of the choices does not matter?\newline(b) How many ways can this be done, if the order of the choices matters?
  1. Calculate combinations: Calculate the number of ways to choose 55 objects from 1515 without considering the order (combinations).\newlineReasoning: Use the combination formula nCr=n!r!(nr)!nCr = \frac{n!}{r!(n-r)!}, where nn is the total number of objects and rr is the number of objects to choose.\newlineCalculation: \(15\)C\(5\) = \frac{\(15\)!}{\(5\)!(\(15\)\(-5\))!} = \frac{\(15\)!}{(\(5\)! \cdot \(10\)!)} = \frac{(\(15\) \cdot \(14\) \cdot \(13\) \cdot \(12\) \cdot \(11\))}{(\(5\) \cdot \(4\) \cdot \(3\) \cdot \(2\) \cdot \(1\))} = \(3003\).
  2. Calculate permutations: Calculate the number of ways to choose \(5\) objects from \(15\) considering the order (permutations).\(\newline\)Reasoning: Use the permutation formula \(nPr = \frac{n!}{(n-r)!}\), where \(n\) is the total number of objects and \(r\) is the number of objects to choose.\(\newline\)Calculation: 1515P55 = \frac{1515!}{(15155-5)!} = \frac{1515!}{1010!} = (1515 \times 1414 \times 1313 \times 1212 \times 1111) = 360360360360.

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