Have a lawn at the front of my house and one at the back. Both lawns are rectangular, with all edges a whole number of metres, and each has an area of 120m², so it takes me the same amount of time to mow both of them. However, the perimeter of the back lawn is 12m greater than the perimeter of the front lawn, so it takes me longer to trim the edges of the back lawn than the front lawn.

What is the difference between the length and the width of the back lawn?

full solution is needed!