In a reservoir, there are (360) litres of water in the first hour and (24) litres decrease each next hour. In which hour will the water be (96) litres?

Correct answer: 12th hour

The water quantities form an arithmetic progression with first term \(a=360\) and common difference \(d=-24\). In the \(n\)th hour, the quantity is \(a_n=360+(n-1)(-24)\). From \(360-24(n-1)=96\), we get \(24(n-1)=264\), so \(n-1=11\) and \(n=12\). Therefore, the water will be 96 litres in the 12th hour. In the 11th hour it would be 120 litres, making it a close but incorrect option. Exam tip: use a negative common difference when a quantity decreases.