The largest possible perimeter is 150 ft. We can write an equation for perimeter by knowing that the perimeter of a rectangle is 2w + 2l.
[tex]2w + 2(w+15)[/tex] = 150
Distribute the 2 and combine like terms.
[tex]2w + 2w + 30 = 150[/tex]
[tex]4w = 120[/tex]
[tex]w=30[/tex]
The length is 15 feet longer than the width, so add 15 to 30 to get 45 feet as the answer.