A logo is formed by three congruent equilateral triangles arranged to form a trapezoid, similar to the figure shown. If the perimeter of the trapezoid is 15 centimeters, what is the side length, in centimeters, of one of the equilateral triangles?

(Note: A diagram showing a trapezoid made of three equilateral triangles, with two shaded, would typically accompany this question on the actual test.)