Below is my question and my professors answer. I do not understand how she gets (B) or (C), if someone could help explain this it would be much appreciated. Thanks!

15. De Trader opens a brokerage account and purchases 300 shares of Internet Dreams at $40 per share. She borrows $4,000 from her broker to help pay for the purchase. The interest rate on the loan is 8%. (LO 34) a. What is the margin in De's account when she first purchases the stock? b. If the share price falls to $30 per share by the end of the year, what is the remain ing margin in her account? If the maintenance margin requirement is 30%, will she receive a margin call? c. What is the rate of return on her investment