The size of stones used in a concrete blend are normally distributed, with a mean diameter of 19.5mm and a standard Deviation of 1.8mm
a) What is the probability that a stone taken at random will have a diameter greater than 18mm
b) The smallest 15.15% of the stones are to be removed by passing them over a sieve. What size should the holes in the sieve be? (ie: below what size will the stones be rejected)