A water tank contains 500 litres of water at 12:00 noon. Every hour a fixed volume of 18.5 litres is drawn out, so that the volumes remaining after each hour form an arithmetic progression.
a. (i) Find the volume of water remaining at 7:00 p.m.
b. (ii) Determine at what time (nearest minute) the volume of water will first fall below 200 litres.