The Harris family needs to purchase topsoil for their landscaping project (measuring 50 ft x 40 ft with a thickness of 6 inches of topsoil/mulch). They have two options:
a. purchasing the soil in 3 ft3 bags priced at $2.25 each with no delivery charge, or
b. purchasing in bulk at $18 per cubic yard with a one-time $50 delivery fee. Which option is the least expensive and by how much? Explain.