Beyond Common Core
Problem 28 of 45s12-028
A quiz has questions, each with answer choices. A student guesses at random on every question. Let be the number correct. What are the mean and standard deviation of ?
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Solution walkthrough1:40
- Binomial: independent guesses, each right with . counts the right ones.
- The mean of a binomial count is : one guess in five is right on average.
- The variance is .
- The standard deviation is the square root: . A typical score is within of .
- Traps: SD is the variance, drops the , and mean counts wrong answers.