The perimeter of a rectangle is 30 inches. if it's length is three times it's width, find the dimensions

Lets x is width
3x is length

P = 2(W+L)
30 = 2(x+3x)
30 = 2(4x)
30 = 8x
so x = 30/8  = 3.75

3x = 3(3.75) = 11.25

width = 3.75 inches and length = 11.25 inches

Let the width be x.

length = 3*width = 3*x = 3x.

Perimeter, P = 2*(l + w),   P = 30 inches. 

P = 2*(l + w)

30 = 2*(3x + x)

30 = 2*(4x)

30 = 8x

8x = 30

x = 30/8 = 15/4 = 3.75

Dimensions, w =x = 3.75 inches, length = 3x =3*3.75 =     11.25 inches.

So dimensions of the rectangle is 11.25 inches by 3.75 inches.

Hope this helps.