If the perimeter of an equilateral triangle is 15 cm, what is the length of each side?

The perimeter of a triangle is the sum of its 3 sides, also, for a equilateral triangle, its sides are equal by definition, so if we call the side length of the triangle x, then we have:
x + x + x = 15
3x = 15
then we divide both sides of the equation by 3
x = 15/3
x = 5
therefore each side of the triangle is 5 cm