Binomial Distribution Math Example 2

Follow the full solution, then compare it with the other examples linked below.

Example 2

hard
A multiple-choice quiz has 1010 questions, each with 44 choices. If a student guesses randomly, what is the probability of getting at least 22 correct?

Solution

  1. 1
    Here n=10n = 10, p=0.25p = 0.25. We need P(Xโ‰ฅ2)=1โˆ’P(X=0)โˆ’P(X=1)P(X \ge 2) = 1 - P(X = 0) - P(X = 1).
  2. 2
    P(X=0)=(100)(0.25)0(0.75)10=(0.75)10โ‰ˆ0.0563P(X = 0) = \binom{10}{0}(0.25)^0(0.75)^{10} = (0.75)^{10} \approx 0.0563.
  3. 3
    P(X=1)=(101)(0.25)1(0.75)9=10ร—0.25ร—(0.75)9โ‰ˆ10ร—0.25ร—0.0751โ‰ˆ0.1877P(X = 1) = \binom{10}{1}(0.25)^1(0.75)^9 = 10 \times 0.25 \times (0.75)^9 \approx 10 \times 0.25 \times 0.0751 \approx 0.1877.
  4. 4
    P(Xโ‰ฅ2)=1โˆ’0.0563โˆ’0.1877=0.756P(X \ge 2) = 1 - 0.0563 - 0.1877 = 0.756.

Answer

P(Xโ‰ฅ2)โ‰ˆ0.756P(X \ge 2) \approx 0.756
The complement method is efficient for 'at least' problems: compute the probability of the complement (fewer than 22) and subtract from 11.

About Binomial Distribution

The probability distribution of the number of successes in nn independent yes/no trials, each with probability pp.

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