Why do this problem?
A real world problem involving conditional probability.
Half the class might work through the problem with a probability of 0.4 of winning rallies against Ivana to see whether calling 9 or 10 isadvantageous, and the other half might work on the problem with a probability of 0.3. Then a class discussion, and drawing tree diagrams of the possibilites, might help everyone to solve the problem for a general value of p and find the critical value.
What are the possible outcomes of the next rally, which is served by Ivana, if my probability of winning the rally is p?
What happens if Ivana loses that rally?
There is a detailed explanation of the solution to this problem in this article