In a building the first floor has (150) windows and each next floor has (7) fewer windows. Which floor will have (52) windows?

Correct answer: 15

The number of windows forms a decreasing AP with \(a=150\) and \(d=-7\). Thus, \(a_n=150+(n-1)(-7)\). Substituting \(a_n=52\) gives \(150-7(n-1)=52\), so \(7(n-1)=98\) and \(n=15\). The 14th floor would have 59 windows and the 16th floor would have 45, so the correct answer is the 15th floor. Exam tip: write the common difference as negative for a decreasing AP.