Math Problem Statement

Suppose a biased coin comes up heads 40% of the time and tails 60% of the time. What is the probability of getting between 2 and 4 heads in 10 flips?

Solution

We can model this problem using the binomial distribution, as we are dealing with a fixed number of independent trials (coin flips), where each trial has two possible outcomes (heads or tails). The probability of getting kk heads in nn flips is given by the binomial probability formula:

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

Where:

  • (nk)\binom{n}{k} is the binomial coefficient, representing the number of ways to choose kk heads out of nn flips: (nk)=n!k!(nk)!\binom{n}{k} = \frac{n!}{k!(n-k)!}.
  • pp is the probability of heads (0.40 in this case).
  • nn is the number of flips (10 in this case).
  • kk is the number of heads we want.

You are asked to find the probability of getting between 2 and 4 heads (inclusive), so we need to calculate the probability for k=2,3,4k = 2, 3, 4 and then sum them up.

Step 1: Calculate for k=2k = 2

P(2 heads)=(102)(0.4)2(0.6)8=10!2!(102)!(0.4)2(0.6)8P(2 \text{ heads}) = \binom{10}{2} (0.4)^2 (0.6)^8 = \frac{10!}{2!(10-2)!} (0.4)^2 (0.6)^8 =45(0.16)(0.0168)=450.002688=0.12096= 45 \cdot (0.16) \cdot (0.0168) = 45 \cdot 0.002688 = 0.12096

Step 2: Calculate for k=3k = 3

P(3 heads)=(103)(0.4)3(0.6)7=10!3!(103)!(0.4)3(0.6)7P(3 \text{ heads}) = \binom{10}{3} (0.4)^3 (0.6)^7 = \frac{10!}{3!(10-3)!} (0.4)^3 (0.6)^7 =120(0.064)(0.028)=1200.001792=0.21446= 120 \cdot (0.064) \cdot (0.028) = 120 \cdot 0.001792 = 0.21446

Step 3: Calculate for k=4k = 4

P(4 heads)=(104)(0.4)4(0.6)6=10!4!(104)!(0.4)4(0.6)6P(4 \text{ heads}) = \binom{10}{4} (0.4)^4 (0.6)^6 = \frac{10!}{4!(10-4)!} (0.4)^4 (0.6)^6 =210(0.0256)(0.0467)=2100.001196=0.25084= 210 \cdot (0.0256) \cdot (0.0467) = 210 \cdot 0.001196 = 0.25084

Step 4: Total Probability

Now, we sum the probabilities for 2, 3, and 4 heads: P(2heads4)=P(2 heads)+P(3 heads)+P(4 heads)P(2 \leq \text{heads} \leq 4) = P(2 \text{ heads}) + P(3 \text{ heads}) + P(4 \text{ heads}) P(2heads4)=0.12096+0.21446+0.25084=0.58626P(2 \leq \text{heads} \leq 4) = 0.12096 + 0.21446 + 0.25084 = 0.58626

Thus, the probability of getting between 2 and 4 heads in 10 flips is approximately 0.586 or 58.6%.

Would you like further details on any step or have any questions?


Here are 5 related questions to extend this topic:

  1. What is the probability of getting exactly 5 heads in 10 flips?
  2. How would the probability change if the coin were fair (50% heads and 50% tails)?
  3. What is the expected number of heads in 10 flips of this biased coin?
  4. How would you calculate the variance and standard deviation for the number of heads in 10 flips?
  5. What is the probability of getting at least 7 heads in 10 flips?

Tip: For binomial probabilities, using a scientific calculator or software tools can speed up the process, especially for larger values of nn and kk.

Ask a new question for Free

By Image

Drop file here or Click Here to upload

Math Problem Analysis

Mathematical Concepts

Probability
Binomial Distribution

Formulas

P(k heads) = \binom{n}{k} p^k (1-p)^{n-k}
Binomial Coefficient: \binom{n}{k} = \frac{n!}{k!(n-k)!}

Theorems

Binomial Probability Theorem

Suitable Grade Level

Grades 9-12