Relationship between events and invoices

Hi, and welcome!

First, for future questions, you should be familiar with the forum's homework policy, and pay special attention to the key role that reproducible examples, called a reprex play in helping to troubleshoot a specific problem.

Here's a checklist for this type of problem.

  1. What is the unknown necessary to calculate the result?
  2. If none of the columns is the unknown, can you calculate the unknown from two or more of them?
  3. Once you've identified the unknown, what function is necessary to calculate the result?
  4. Is the result the same or different if you group the data by other variables.
  5. (Possibly extra credit to your assignment.) What is the probability that the observed differences are due to chance?