we find volume by multiplying length x width x height, or depth in this case.
v = lwh. How do we "solve" this for width? We divide both sides by l and h. We end up with w = v/lh
Plugging in what we have, w = 6510/(40 x 10 1/2) = 6510/420 = 15 1/2.
So, the hole is 15 1/2 feet wide.
Hope this helps!