Math Problem Statement

A recent study has shown that a new treatment cures a certain disease 81% of the time. A random sample of 11 patients undergoing this treatment is chosen. Find the probability that 8 or 9 of them are cured.

Do not round your intermediate computations, and round your answer to three decimal places.

Solution

This problem can be approached using the binomial probability formula, which is:

P(X=k)=(nk)pk(1p)nkP(X = k) = \binom{n}{k} p^k (1 - p)^{n - k}

where:

  • n=11n = 11 (the number of patients),
  • kk is the number of patients we want to find probabilities for (8 and 9, in this case),
  • p=0.81p = 0.81 (the probability of a single patient being cured),
  • (nk)\binom{n}{k} is the binomial coefficient, which calculates the number of ways to choose kk cured patients out of nn total patients.

To solve the problem, we need to calculate P(X=8)P(X = 8) and P(X=9)P(X = 9), then add these probabilities together.

Step 1: Calculate P(X=8)P(X = 8)

  1. Binomial coefficient: (118)=11!8!(118)!=11!8!3!=165\binom{11}{8} = \frac{11!}{8!(11 - 8)!} = \frac{11!}{8! \cdot 3!} = 165
  2. Probability term for k=8k = 8: P(X=8)=(118)(0.81)8(0.19)3P(X = 8) = \binom{11}{8} (0.81)^8 (0.19)^3 P(X=8)=165(0.81)8(0.19)3P(X = 8) = 165 \cdot (0.81)^8 \cdot (0.19)^3

Step 2: Calculate P(X=9)P(X = 9)

  1. Binomial coefficient: (119)=11!9!(119)!=11!9!2!=55\binom{11}{9} = \frac{11!}{9!(11 - 9)!} = \frac{11!}{9! \cdot 2!} = 55
  2. Probability term for k=9k = 9: P(X=9)=(119)(0.81)9(0.19)2P(X = 9) = \binom{11}{9} (0.81)^9 (0.19)^2 P(X=9)=55(0.81)9(0.19)2P(X = 9) = 55 \cdot (0.81)^9 \cdot (0.19)^2

Step 3: Add P(X=8)P(X = 8) and P(X=9)P(X = 9)

Now, calculate both probabilities and add them up:

P(8 or 9 cured)=P(X=8)+P(X=9)P(8 \text{ or } 9 \text{ cured}) = P(X = 8) + P(X = 9)

After performing these calculations (using exact values for accuracy in intermediate steps), round the final answer to three decimal places.

Let's calculate it fully:

Calculation:

  1. P(X=8)=165(0.81)8(0.19)30.233P(X = 8) = 165 \cdot (0.81)^8 \cdot (0.19)^3 \approx 0.233
  2. P(X=9)=55(0.81)9(0.19)20.274P(X = 9) = 55 \cdot (0.81)^9 \cdot (0.19)^2 \approx 0.274

P(8 or 9 cured)0.233+0.274=0.507P(8 \text{ or } 9 \text{ cured}) \approx 0.233 + 0.274 = 0.507

Final Answer:

The probability that exactly 8 or 9 out of the 11 patients are cured is approximately:

0.507\boxed{0.507}

Would you like further details or have questions about any steps?


  1. What is a binomial probability distribution?
  2. How is the binomial coefficient calculated?
  3. What does it mean when we add probabilities for two values?
  4. Can this process be applied to other probabilities?
  5. What would happen if we increased the probability of success to 90%?

Tip: In binomial problems, always check if events are independent and only have two outcomes per trial.

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Math Problem Analysis

Mathematical Concepts

Probability
Binomial Distribution

Formulas

Binomial probability formula: P(X = k) = (n choose k) * p^k * (1 - p)^(n - k)
Binomial coefficient formula: (n choose k) = n! / (k!(n - k)!)

Theorems

Binomial Theorem

Suitable Grade Level

Grades 11-12