A staircase has (30) bricks in the bottom row and (2) fewer bricks in each upper row. How many bricks are there in (10) rows?

Correct answer: 210

The numbers of bricks form a decreasing AP: \(30, 28, 26, \ldots\). Here, \(a=30\), \(d=-2\), and \(n=10\). The 10th row has \(a_{10}=30+9(-2)=12\) bricks. Therefore, \(S_{10}=\frac{10}{2}(30+12)=210\). Hence, 210 is correct. The value 220 would result from incorrectly taking 14 as the last term; 14 is the 9th term. Exam tip: find the first and last terms, then use \(S_n=\frac{n}{2}(a+l)\).