At a high school, 20% of statistics students retake their 3 week test. There are 15 students in Mr. Smith’s class.

Question

Find the probability that exactly four of them retake this test.

Answer 1

Since each of the students re-taking the test is assumed independent can use binomial formula to compute probability:

x n-x

P(x successes in n tries) = n! p (1-p)

--------

x! (n-x)!

Here p = 0.2, n = 15 and x = 4. plug values into the equation and solve.