The length of a is 3 times its width if the perimeter is 400ft what is the length and width of the fence

Width is x

length is 3x

(Don't forget that perimeter has 2 sides of width and 2 sides of length)

so perimeter is 400 = 2 (x) + 2 (3x)

400 = 2x+6x

400 = 8x

so x = 50

then the length of the fence is 50ft

the width of the fence is 150ft