The initial margin is 50%, so the broker-dealer will lend the investor $10,000, and the investor must put up $10,000 of their own money. The maintenance margin is 25%, so the minimum value of the account must be $5,000 (25% of $20,000).

If the stock's value falls to $11,100, the amount of equity in the account is $1,100 ($11,100 value of the stock - $10,000 loan from the broker).

The margin as a percentage of the current value of the stock is 9.99% ($1,100 / $11,100).

Since this is less than the 25% maintenance margin requirement, there is a margin call to bring the margin balance up to 25% of the value of the shares, which is $1,675 ($11,100 × 25% - $1,100).

Therefore, the investor must add $1,675 to the margin account to meet the maintenance margin requirement. If this is not provided, the broker may sell some or all of the securities without notification to bring the account back up to minimum margins.