Math Problem Statement

A student must answer exactly eight questions out of ten on a final examination. In how many ways can she choose the questions to answer if she must answer at least three of the last five questions and at most four of the first five.

Solution

To solve this problem, we need to calculate the number of ways the student can select 8 questions while satisfying the given constraints.


Constraints:

  1. She must answer at least 3 of the last 5 questions.
  2. She can answer at most 4 of the first 5 questions.

Step-by-step Solution:

  1. Total choices:

    • First 5 questions → Let xx be the number of questions answered from the first 5.
    • Last 5 questions → She must answer 8x8 - x questions from the last 5.
  2. Constraint on the last 5 questions:

    • At least 3 questions must be answered from the last 5, so 8x38 - x \geq 3, which implies x5x \leq 5.
  3. Constraint on the first 5 questions:

    • At most 4 questions can be answered from the first 5, so x4x \leq 4.

Combining these two constraints, x4x \leq 4.

  1. Possible values of xx:
    • Since 8x58 - x \leq 5 (the maximum number of questions available in the last 5), x3x \geq 3.
    • Thus, xx can be 33 or 44.

Case Analysis:

For each xx, calculate the number of ways to choose the questions.

Case 1: x=3x = 3 (3 from the first 5, 5 from the last 5)

  • Number of ways to choose 3 questions from the first 5: (53)=10\binom{5}{3} = 10
  • Number of ways to choose 5 questions from the last 5: (55)=1\binom{5}{5} = 1
  • Total for this case: 10×1=1010 \times 1 = 10

Case 2: x=4x = 4 (4 from the first 5, 4 from the last 5)

  • Number of ways to choose 4 questions from the first 5: (54)=5\binom{5}{4} = 5
  • Number of ways to choose 4 questions from the last 5: (54)=5\binom{5}{4} = 5
  • Total for this case: 5×5=255 \times 5 = 25

Final Answer:

Add the totals from all cases: 10+25=3510 + 25 = 35

The student can choose the questions in 35 ways.


Do you want a detailed breakdown of any specific step or additional explanation?


Related Questions:

  1. What happens if the student must answer at least 4 of the last 5 questions instead of 3?
  2. How does the number of combinations change if the student is allowed to answer all 10 questions?
  3. What is the total number of ways to select 8 questions without any restrictions?
  4. How would the result change if the student could answer at most 3 of the first 5 questions?
  5. What if the student must answer exactly 2 questions from the last 5?

Tip:

When dealing with multiple constraints in combinatorics, break the problem into smaller cases and systematically calculate for each case.

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Math Problem Analysis

Mathematical Concepts

Combinatorics
Binomial Coefficients
Constraints in Selection

Formulas

nCr = n! / (r! * (n - r)!)

Theorems

Combination Rule in Combinatorics

Suitable Grade Level

Grades 10-12