The amount of milligrams (mg) of sodium in the water in a certain neighbourhood of Winnipeg has a mean of 60 mg/l, and a standard deviation of 10 mg/l. Assume the variable is normally distributed.

a) If one sample of water is selected from the neighbourhood, what is the probability that the sodium content will be more than 70 mg/l?

b) If four samples of water from the neighbourhood are selected, what is the probability that their mean will be larger than 70 mg/l?

c) Why is the probability for part (a) greater than that for part (b)?

d) If 16 samples of the water from the neighbourhood is selected, what is the probability that their total sodium content will be larger than 1120 mg/l?