You find a certain stock that had returns of 9 percent, 16 percent, 21 percent, and 17

Question:

You find a certain stock that had returns of 9 percent, −16 percent, 21 percent, and 17 percent for four of the last five years. If the average return of the stock over this period was 11.3 percent, what was the stock’s return for the missing year? What is the standard deviation of the stock’s return?