A water tank is filled by adding water daily according to the following plan: 600 litres are poured in on Day 1 and on each subsequent day the amount poured in is 60 litres more than the previous day.
(i) Explain why the daily amounts of water poured form an arithmetic progression. State its first term and common difference.
(ii) Write the expression for the amount of water poured on Day n.
(iii) Without numerical substitution, describe how you would find the total volume poured in during the first n days.