Length = 20 ft
If he uses 60 ft of fencing to line his rectangular backyard, then the perimeter is 60 ft.
Perimeter formula is ((Width × 2) + (Length × 2))
10 × 2 = 20
20 × 2 = 40
40 + 20 = 60
I think it is Adjacent and Supplementary
6.4 is the answer hope this helps