Can you solve the river fence puzzle? This math problem has very practical applications in the real world, and can be solved with both geometry and calculus. Imagine you have 100 units of fencing material. You must box off an area of land next to a river, with the river acting as one side of the box. How can you arrange the 100 units of fencing to box off the largest possible area of land next to the river? If you think you have the correct answer, how do you know it's correct?