We were told that 
1 pound of active ingredient is required per 1000 square feet. 
Given that lawn is 10,000 square feet in size, let x represent the amount of product needed for the lawn. We would set up the equation as shown below
1 = 1000
x = 10000
By cross multiplying, it becomes
x * 1000 = 1 * 10000
1000x = 10000
x = 10000/1000
x = 10 
10 pounds of active ingredient would be needed to treat the entire lawn