Are you sure this is how the question is written (i.e. asking for the exact average cost)? I found a version of this question that goes like this:
An early purchase plan for seats at the Rio Olympic Games offered a seat at 9 events for $750 deposit and $228 per month for 12 months. Under this plan, the mean cost of a seat at any one of these events is closest to:
In this case the answer would indeed be $400 as the average cost of one event is about $387.33 and is closest to $400.