First, you multiply 15•2. This gives you the total excess length of the two longer sides.
Next, subtract that (30) from the total perimeter (90) ⇒ 90-30
Now all you have left is the total length of the two short sides, and the short side bit of the long side. (It is now a square because you removed the extra fifteen on the two long sides if that makes sense).
After that, divide the 60 (90-30) by four. This gives you the length of each side of the square we have now (15).
So now you know that the two short sides are 15. And if you add 15 to the two long sides (15+15), then now you know the long sides are 30.
Therefore the answer is the dimensions are 30 feet long and 15 feet wide.