The depth varies quadratically with the width, so the quadratic equation has the form
d = kw^2
where d is the depth, w is the width, and k is a constant.
We can use the given information to find k.
At a width of 200 m, the depth is 20 m.
20 = k(200)^2
k = 0.0005
The equation of the depth is
d = 0.0005w^2
Now use a width of 1000 m to find the depth at the greatest width.
d = 0.0005w^2
d = 0.0005(1000)^2
d = 500
Answer: 500 m
