Question

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

Answer 1

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. 

Answer 2

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.