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

If the length is 3 parts and the width is 1 part, then the total is 4 parts.

So you do 30 ÷ 4 (parts) = 7.5 = 1 part. 

Because the width is 1 part, you leave it as 7.5 inches.
Obviously, a rectangle has 4 sides, so that means it has two sides that are the width. So split it into two, and you get 3.75 inches as the width (for one side). 

Now, the length is 3 × 1 part, so you do 7.5 (1 part) × 3 = 22.5 inches.
Same thing applies here, there are going to be 2 parts as the length, so split 22.5 into two to get 11.25 inches.

Length = 11.25 inches
Width = 3.75 inches

We can now check if this is correct by adding up everything to find the perimeter. 

So: 3.75 + 3.75 + 11.25 + 11.25 = 30 inches, so the answers are correct.