In September 2018, TD Bank issued the new Series 20 preferred shares. In September, you bought 100 shares for $25 per share, in November you bought 80 more shares for $24 per share and in December bought another 220 shares for $21 per share. 
 The price of TD shares recently fell to $20/share so you are considering buying more shares. How many more shares would you need to buy to reduce the overall average price paid to $21.25/share?  
 You did not buy any more shares because you thought the share price might fall even further. However, a few weeks later, you bought another 500 shares which reduced the overall average price paid to $20.75. What price per share did you pay for the 500 shares?