First:

A company purchases a billboard on the side of the highway for $10,000. For each person that calls the number on the billboard and fills out an application, the company receives $100. 15% of people who call the number actually fill out an application. What is the minimum number of calls that need to be made so that the company can make a profit off the billboard?

Second:

A company sends out an email to 1,000,000 people to register for an event. The email has a 8% open rate, 5% click-through rate, 200 registrations at $10/registration. What is the cost per click?

I had a hard time with these two at a recent interview.

Thanks in advance.