A customer buys 100 shares of ABC at $30 as the initial transaction in a new margin account. Subsequently, ABC rises to $40 per share in the market. What is the account's equity after the change in market value

Account's Equity:

Investment in ABC = $4,000 ($40 x 100)

This is made up of initial investment of $3,000 and Unrealized Gain of $1,000 ($10 x 100) for the appreciation in market value.

Explanation:

The account's equity is the share in ownership rights of the customer in the margin account.  According to investopedia.com, "A margin account is a brokerage account in which the broker lends the customer cash to purchase stocks or other financial products.  The loan in the account is collateralized by the securities purchased and cash, and comes with a periodic interest rate."