Suppose that the size of pebbles in a river bed is normally distributed with mean 12.1mm and

Question:

Suppose that the size of pebbles in a river bed is normally distributed with mean 12.1mm and standard deviation 3.2 mm. A random sample of 9 pebbles are measured. Let denote the average size of the sampled pebbles.
(a) What is the of ?
(b) What is the probability that is smaller than 10 mm?
(c) What percentage of the pebbles in the river bed are of size smaller than 10 mm? Distribution
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