Math Problem Statement

Let X be the number of times a "2" is rolled when a fair, four-sided die is rolled 13 times.

Then X has a Binomial distribution where n = 13 and p = 0.25. What is the probability that we roll a "2" at most 2 times.

Solution

To solve this problem, we use the Binomial distribution formula:

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

where:

  • n=13n = 13 is the number of trials,
  • p=0.25p = 0.25 is the probability of rolling a "2" on a single roll,
  • kk is the number of successes (number of times "2" is rolled),
  • (nk)=n!k!(nk)!\binom{n}{k} = \frac{n!}{k!(n-k)!} is the number of combinations.

The probability of rolling a "2" at most 2 times is:

P(X2)=P(X=0)+P(X=1)+P(X=2).P(X \leq 2) = P(X = 0) + P(X = 1) + P(X = 2).

Step 1: Compute individual probabilities

For X=0X = 0:

P(X=0)=(130)(0.25)0(0.75)13=11(0.75)13.P(X = 0) = \binom{13}{0} (0.25)^0 (0.75)^{13} = 1 \cdot 1 \cdot (0.75)^{13}.

P(X=0)=(0.75)130.01336.P(X = 0) = (0.75)^{13} \approx 0.01336.

For X=1X = 1:

P(X=1)=(131)(0.25)1(0.75)12=130.25(0.75)12.P(X = 1) = \binom{13}{1} (0.25)^1 (0.75)^{12} = 13 \cdot 0.25 \cdot (0.75)^{12}.

P(X=1)=130.25(0.75)120.05782.P(X = 1) = 13 \cdot 0.25 \cdot (0.75)^{12} \approx 0.05782.

For X=2X = 2:

P(X=2)=(132)(0.25)2(0.75)11=13122(0.25)2(0.75)11.P(X = 2) = \binom{13}{2} (0.25)^2 (0.75)^{11} = \frac{13 \cdot 12}{2} \cdot (0.25)^2 \cdot (0.75)^{11}.

P(X=2)=780.0625(0.75)110.14403.P(X = 2) = 78 \cdot 0.0625 \cdot (0.75)^{11} \approx 0.14403.

Step 2: Add probabilities

P(X2)=P(X=0)+P(X=1)+P(X=2).P(X \leq 2) = P(X = 0) + P(X = 1) + P(X = 2).

P(X2)0.01336+0.05782+0.14403=0.21521.P(X \leq 2) \approx 0.01336 + 0.05782 + 0.14403 = 0.21521.

Final Answer:

The probability of rolling a "2" at most 2 times is approximately:

0.2152(rounded to 4 decimal places).\boxed{0.2152 \, \text{(rounded to 4 decimal places)}}.


Would you like further details or a breakdown of any step? Here are some related questions:

  1. How is the binomial distribution used in real-world problems?
  2. What does the shape of the binomial probability mass function look like for n=13n = 13 and p=0.25p = 0.25?
  3. How do we calculate probabilities for P(X>2)P(X > 2) using the complement rule?
  4. What is the expected value and standard deviation for XX in this distribution?
  5. Can we approximate this binomial distribution with a normal distribution?

Tip: When dealing with cumulative probabilities in binomial distributions, consider using statistical tables or software for efficiency.

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Math Problem Analysis

Mathematical Concepts

Binomial Distribution
Probability Theory
Combinatorics

Formulas

P(X = k) = C(n, k) * p^k * (1 - p)^(n - k)
P(X ≤ k) = P(X = 0) + P(X = 1) + ... + P(X = k)

Theorems

Binomial Distribution Theorem

Suitable Grade Level

Grades 10-12