H. standard deviation of 2.5 mm Suppose that the size of pebbles in a riverbed is normally distributed with a mean of 12 mm and a ") Find the probability that a randomly selected pebble in this riverbed has size less than 9 mm. (4 pts) b) Consider the probability that the mean size of 15 randomly selected pebbles is less than 9 mm. Without doing any computations, is this probability less than, greater than or the same as the probability you found in part a? (2 pts) c) Suppose a little boy randomly chooses 20 of these pebbles to throw into the river, State the model that can be used to describe the distribution of the mean size of the 20 pebbles. (4 pts) d) Find the probability that the mean size of 20 random pebbles is greater than 13.8 mm (4 pts) e) Find the 70" percentile of the distribution in part c. (4 pts)