A farmer wishes to concrete his rectangular feed lot, which measures 120 ft by 180 ft. He wants to have a base of 4 in. of gravel covered with 3 1/2 in. of concrete. a. How many cubic yards of each material must he purchase?

b. What is his total materials cost? Concrete costs $50/yd3 delivered, and gravel costs $5/ton delivered. (1 yd3 of gravel weighs approximately 2500 lb.)
Please answer step-by-step thank you.​