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counting and probability senior

Problem

In a class of 28 students, the teacher selects four people at random to participate in a geography contest. What is the probability that this group of four students includes at least two of the top three geography students in the class? Express your answer as a common fraction.
Solution
We count the number of ways to choose a group of four students including at least two of the top three geography students. This is just , since we can choose either 2 or 3 of the top students to be in this group. In all, there are groups of four students. Thus our desired probability is .
Final answer
\frac{37}{819}