You find a certain stock that had returns of 12 percent, 21 percent, 27 percent, and 18

Question:

You find a certain stock that had returns of 12 percent, −21 percent, 27 percent, and 18 percent for four of the last five years. If the average return of the stock over this period was 10 percent, what was the stock’s return for the missing year? What is the standard deviation of the stock’s returns?