The total perimeter of 300 yards is comprised of 2 x Length and 2 x Width.
Thus 1 x Length + 1 x Width = 300 / 2 = 150 yards.
If Length = 2 x Width, then the ratio of length to width is 2 : 1
2 + 1 = 3
Divide 150 by 3 to give 50 and then assign the ratio
Thus Width = 1 x 50
Length = 2 x 50 = 100
Thus 1 x Length + 1 x Width = 300 / 2 = 150 yards.
If Length = 2 x Width, then the ratio of length to width is 2 : 1
2 + 1 = 3
Divide 150 by 3 to give 50 and then assign the ratio
Thus Width = 1 x 50
Length = 2 x 50 = 100