A landscaper has 2500 square feet of grass land in a customer’s backyard where they need to cut grass using either a heavy-duty yard machine or a gas lawn mower. For each square foot of land, the heavy-duty yard machine requires 2 seconds while the gas lawn mower takes 5 seconds. Per square foot, the heavy-duty yard machine produces 12 units of pollution and requires 3 units of fuel. Conversely, the gas lawn mower generates 10 units of pollution for each square foot and requires 2 units of fuel. There are 450 units of fuel available and the landscaper can produce no more than 2000 units of pollution. For how many square feet should each machine be used to minimize the time required to cut this customer’s backyard?