The width is 12.5 and the length is 37.5.
In order to find this, set the width as x and the length as 3x. Now we can multiply them together using the rectangle perimeter formula.
2L + 2W = P
2(x) + 2(3x) = 100
2x + 6x = 100
8x = 100
x = 12.5
Now we know the width is 12.5 and we can find the length by multiplying by 3, which is 37.5.