Question

what is the length of $overline{mn}$?

Explanation:

Step1: Identify coordinates

The coordinate of point M is 400 and the coordinate of point N is 800.

Step2: Calculate distance

The length of $\overline{MN}$ is found by subtracting the smaller coordinate from the larger one. So, $800 - 400$.
$800-400 = 400$

Answer:

400