Suppose you shorted 100 shares of AAPL at $180 with 80% margin. How much equity you need to start the trade. Suppose AAPL went up to $250, the initial margin is 50%, and the maintenance margin is 30%, do you receive a margin call? If you do, then how much money you need to put in your account to pull your margin back to the initial margin. Alternatively, if you choose to buy back some of the shares, then how many shares do you need to buy back?

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