To find the perimeter of Δ XYZ when its longest side is 20, you can set up proportions to find the lengths of the sides, since the sides of similar triangles
(The two triangles are not to scale. The one on the right is drawn twice the size of the one on the left but actually is just half it's size.)
Also, the order of the letters in the congruency statement (triangle ABC is congruent to triangle XYZ) is important. If they had stated that triangle ABC is congruent to triangle ZYX, the relationship would not be 4/8 but 4/16. This would have resulted in entirely different answers. Instead of triangle XYZ being 1/2 the size of ABC, it would have been 1/4 it's size.
