Question

Ben is a member of a class with 25 students that meets five days per week. Each day class meets, a student in the class is randomly selected to explain how to solve a homework problem. Once a student has been selected, he or she will not be selected again that week. If Ben was not one of the four students selected earlier in the week, what is the probability that he will be picked on the last day of class during the week? 

Answer 1

Solution:

We need to find a probability that Ben is selected, given that 4 students have already been selected.

Out of 25 students only 21 have not been asked yet, so the probability is:

$P(Ben|4sel) = \frac{1}{21} \approx 0.0476$

Answer:

$\frac{1}{21} \approx 0.0476$