So the perimeter of the first triangle is 30; since you are given each side you can set up 3 ratios to find the unknown side lengths. Thus side 5 would correspond to 2.5 Side 12 would correspond to 6 And side 13 would be 6.5
Another way to look at it is that since the perimeter of triangle 2 decreases by half (from 30 to 15), you can assume all side will also decrease by half
