The length around the circle is called the circumference, the length aross the circle is the diameter and half of that is called the radius.
Lets set up the relationship of the length to width:
l = 3w
Using this, we can make all the variables the same by substitution:
2l + 2w = perimeter
2l + 2w = 72
2(3w) + 2w = 72
6w + 2w = 72
8w = 72
w = 9
The width is 9, to the length must be 9*3, or 27.
Hope this helped!
For 28(3/4), the answer is 21
