A reservoir has (500) litres of water on the first day and (25) litres less each next day. How much water remains on the (12)th day?

Correct answer: (225)

The direct answer is B, 225 litres. The amounts form a decreasing arithmetic progression because the same quantity, 25 litres, is removed every day. The first term is \\(a=500\\), the common difference is \\(d=-25\\), and the required term is the 12th term. There are 11 decreases before day 12, so \\(a_{12}=a+(12-1)d=500+11(-25)=500-275=225\\). Option B is therefore correct. Option A, 200, would result from using too many decreases or making another arithmetic error. Option C, 250, is not the result after eleven reductions; it is too large. Option D, 275, also ignores part of the required decrease. The minus sign is essential because the quantity goes down, not up. Memory cue: for the nth term, use first term plus \\(n-1\\) times the common difference; count the changes before the requested day.