A circular concrete slab is a cylinder. Vol = pi%2Ar%5E2%2Ah in this cylinder: r = 18%2F2 = 9 ft h = 3%2F12 = .25 ft :
Find the volume in cu/ft: V = pi%2A9%5E2%2A.25 V = 63.61725 cu/ft : Change to cubic yds; (1 cu/yd = 27 cu/ft) and multiply by $40: Cost = 63.61725%2F27 * 40 Cost = $94.25
In practical terms , they may require you to buy 3 cu/yds , which is $120