If two triangles are similar, then the ratio of their corresponding sides is constant. The perimeter of a traingle is the sum of the length of each side. This means that the ratio of the perimeter of both triangles is also constant.
Let x represent the perimeter of the smaller triangle. Given that the perimeter of the larger triangle is 120 cm, it means that
3/5 = x/120
By cross multiplying, it becomes
5 * x = 3 * 120
5x = 360
x = 360/5
x = 72
Perimeter of the smaller triangle is 72 cm
