Can you please solve this problem step by step and show the used formula?

6) An investor buys $8,000 worth of a stock priced at $40 per share using 50% initial margin. The broker charges 6% on the margin loan and requires a 30% maintenance margin. In 1 year the investor has interest payable and gets a margin call. At the time of the margin call the stock's price must have been less than A) $20 B) $29.77 C) $30.29 D) $32.45