A reservoir has (210) litres of water on the first day and (12) litres less each next day. On which day will (66) litres remain?

Correct answer: 13

The amount of water forms a decreasing AP with \(a=210\) and \(d=-12\). On the \(n\)th day, \(a_n=210+(n-1)(-12)\). Solving \(210-12(n-1)=66\) gives \(n-1=12\), so \(n=13\). Therefore, 66 litres remain on the 13th day. On the 12th day, the amount is 78 litres, so option 12 is not correct. Exam tip: use a negative common difference \(d\) for a decreasing AP.