Answer: Length of fence along the diagonal is 28.3 feet.
Step-by-step explanation:
Since we have given that
The Blackburn family has a square field ,
Area of square = 40000 ft square
As we know the formula,

But Blackburn wants to put a fence diagonally through the field ,
As we know the formula for "Diagonal of side" i.e.

Hence, length of fence along the diagonal is 28.3 feet.