perimeter of the top of desk is 54 inches. if the length of the desk is 15 inches, what is the width of the desk
Width is 12 inches. Perimeter is add all sides so 15+15 is 30. Next you subtract 30 from 54 which is 24. So with two sides remaining you divide by 2 which is 12, the width is 12 inches.
12 inches because if you know one side is 15, the side opposite of it must be 15 as well. 15+15=30. Then you are left with 24. 24 divided by 2 is 12.