For the footing, each linear foot requires 1 cubic foot of concrete. You have 60 linear feet of footer, hence 60 cubic feet required there. The slab is 240 square feet, so for a 4 inch thick slab you would need an additional 80 cubic feet of concrete, for a total of 140 cubic feet, which is a little over 5 cubic yards. Due to spillage and waste, best to order 6 yards of concrete. Unless of course your slab is not 4 inches thick, in which case you need to modify the computations.