In a building, the first floor has 260 windows and each next floor has 18 fewer windows. Which floor will have 62 windows?

Correct answer: 12

The governing concept is the nth term of an arithmetic progression with a negative common difference. The first floor gives a = 260, and because each successive floor has 18 fewer windows, d = −18. For the nth floor, a_n = a + (n − 1)d. Setting the term equal to 62 gives 62 = 260 − 18(n − 1). Hence −198 = −18(n − 1), so n − 1 = 11 and n = 12. A direct check confirms this: from the first to the twelfth floor there are 11 decreases, and 260 − 11 × 18 = 260 − 198 = 62. Thus option C is correct. The other answers result from counting 12 decreases instead of 11 or mishandling the negative difference.