Find the minimum value of the average cost for the given cost function on the given intervals. C(x) =x +31x+128 (a) 15x5 10 (b) 10 Sxs 20 (a) The minimum value of the average cost over the interval 1 S x$ 10 is (Round to the nearest tenth as needed. Do not include the $ symbol in your answer.) (b) The minimum value of the average cost over the interval 10 s x $ 20 is (Round to the nearest tenth as needed. Do not include the $ symbol in your answer.)