Math Problem Statement

it is given that the permutation P(n, r) = 240 and combination C(n,r) = 120 find the value of r

Solution

We are given the following two equations:

P(n,r)=240andC(n,r)=120P(n, r) = 240 \quad \text{and} \quad C(n, r) = 120

We know the following formulas for permutations and combinations:

  1. Permutation:
    P(n,r)=n!(nr)!P(n, r) = \frac{n!}{(n - r)!}
  2. Combination:
    C(n,r)=n!r!(nr)!C(n, r) = \frac{n!}{r!(n - r)!}

To find the value of rr, let's divide the permutation formula by the combination formula, which eliminates n!n! and simplifies the equation:

P(n,r)C(n,r)=n!(nr)!n!r!(nr)!=r!\frac{P(n, r)}{C(n, r)} = \frac{\frac{n!}{(n - r)!}}{\frac{n!}{r!(n - r)!}} = r!

Thus:

240120=r!\frac{240}{120} = r!

Simplifying:

2=r!2 = r!

Now, we solve for rr. The factorial of rr that equals 2 is:

r=2r = 2

Thus, the value of rr is 2.

Would you like further details or explanations? Here are 5 related questions for further understanding:

  1. How do you calculate n!n! for any given nn?
  2. Can you explain why we divided P(n,r)P(n, r) by C(n,r)C(n, r)?
  3. What is the difference between permutations and combinations?
  4. How does the factorial operation impact larger values of rr?
  5. How would the equations change if P(n,r)P(n, r) and C(n,r)C(n, r) were larger numbers?

Tip: The division trick for simplifying P(n,r)P(n, r) and C(n,r)C(n, r) is useful when you need to eliminate complex factorials.

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Math Problem Analysis

Mathematical Concepts

Permutations
Combinations
Factorials

Formulas

Permutation: P(n, r) = n! / (n - r)!
Combination: C(n, r) = n! / (r!(n - r)!)

Theorems

Factorial Theorem

Suitable Grade Level

Grades 9-11