The perimeter of an equilateral triangle ABC is 24 cm, find the length of the midline of this triangle.

The middle line is parallel to one of the sides of the triangle and is equal to half of it. To find the length of the midline of a triangle ABC, it is enough to know the length of one of its sides, and since this triangle is regular (equilateral), all of its sides and all midlines are equal.
Let’s find the length of the side of the triangle ABC:
a = P / 3,
where a is the length of the side, P is the perimeter of the triangle ABC.
a = 24/3 = 8 (cm).
The midline of an equilateral triangle ABC is equal to:
l = a / 2 = 8/2 = 4 (cm).
Answer: l = 4 cm