The perimeter of any shape is equal to the sum of the length of the sides of the shape. For a triangle, it would be the sum of the three side lengths. To determine the value of the perimeter, of course, we need the length of these three. By using ratio and proportion, we would be able to determine the length of the sides assuming that the two triangles are similar. We do as follows:
10 / 20 = 50 / x
where x is the length of the longest side of the larger triangle
x = 100
So, the length of the sides of the larger triangle are 50, 50 and 100. The perimeter would be
Perimeter = 50 + 50 + 100 = 200 inches
