At a coal mining facility, a tipple is calibrated to load the weight of μ = 18.00 tons of coal into each hopper, with a standard deviation of σ = 0.61 tons. Find the probability that a random sample of 20 hoppers will yield a mean weight between 17.80 tons and 18.20 tons. 4pt a) using probability notation, construct the probability expression being asked for (be sure to include a random variable in your proper probability notation) 3pt b) what assumption is required to actually find the probability in part (a) above