During the flu season, 1/5
of the total population is infected. A big
school decides to examine its students for
flu infection. What is the probability that when the
students are examined one at a time, the first student to have the flu is the fourth student
examined? Hint: Think of geometric random variables.

Question

During the flu season, 1/5
of the total population is infected. A big
school decides to examine its students for 
flu infection. What is the probability that when the
students are examined one at a time, the first student to have the flu is the fourth student
examined? Hint: Think of geometric random variables.

dumbcow · Answer

http://en.wikipedia.org/wiki/Geometric_distribution p = 1/5 $$P(k=4) = (\frac{4}{5})^{3} \frac{1}{5}$$

anonymous · Answer

Can you explain step by step how you did this?

kropot72 · Answer

The explanation at the link given by @dumbcow looks very good. Please look at it carefully @anneflys.