Mark values his drum set at $800 and ella values her guitar at $1,000. suppose that mark trades his drum set for ella's guitar.

If they made this trade, it would make sense that Mark would also have to give Ella $200. This is because the difference in value is that amount in the favor of Ella. You can see that by subtracting the two values from each other.

$1000 - $800 = $200