9. What will be the investor’s return if a stock rises by 7% if purchased on 50% margin?
10. What will be the investor’s return if a stock falls by 9% if purchased on 60% margin?
11. What is the investor’s profit on a short sale at $45 if covered at a price of $30?
I suggest you work out an example. Buy $100 of stock at 50% margin means $50 comes out of the investor's pocket, the other $50 is borrowed. The stock rises to $107, the stock is sold, the loan is repaid plus interest (of amount r). So investor is left over with ($57-r). Since the initial investment was 50, the rate of return is (57-r)/50. If the turn-around time is short (say one day), then r will be very small and the rate of return will be nearly 14%. Not bad for one day.
Take it from here.