Given that the line segment AB = 10cm, point C is any point on the line segment AB, m and N are the midpoint of AC and BC respectively, the length of Mn can be obtained

Given that the line segment AB = 10cm, point C is any point on the line segment AB, m and N are the midpoint of AC and BC respectively, the length of Mn can be obtained

Because AB = 10, so AC + BC = 10, and because m and N are the midpoint, so am = cm, BM = cm, so Mn = 5cm
You can just draw a picture below