In the Netherlands, healthy males between the ages of 65 and 79 have a distribution of serum uric acid levels that is approximately normal with mean = 420 mol/l and standard deviation = 80 mol/l.

a. What proportion of the males has a serum uric acid level between 350 and 460 mol/l?

b. What proportion of samples of size 10 have a mean serum uric acid level between 350 and 460 mol/l?

c. What proportion of samples of size 20 have a mean serum uric acid level between 350 and 460 mol/l?

d. Construct an interval that encloses 95% of the means of samples of size 20. Which would be shorter, a symmetric interval or an asymmetric one?