if the pond were a rectangle, then:
length=15 m
width=6 m
We would have to calculate the perimeter of this rectangle.
Perimeter of a rectangle=2(length)+2(width)
Perimeter of this rectangle=2(15 m)+2(6 m)=30 m+12 m=42 m
We need 42 m and I only have 40 m, therefore, it would not be enough for fencing the pond.
