An investor accumulated 1800 shares of Microtel Corporation over a period of several months. She bought 1000 shares at $15.63, 500 shares at $19.00, and 300 shares at $21.75. What was her average cost per share? (Note: Investors who purchase shares in the same company or the same mutual fund at more than one price must eventually do this calculation. Tax rules require that the capital gain or loss on the sale of any of the shares be calculated using the weighted-average price paid for all of the shares, rather than the particular price paid for the shares actually sold.)