There are 10 rows of seats in a lecture theater, the first row has 20 seats, and each subsequent row has 2 more seats than the previous row. How many seats are there in this classroom?
一个阶梯教室共有10排座位，第一排有20个座位，往后每一排都比前一排多2个座位，这个教室有多少个座位？
<gadget id="calculator">20 + 18</gadget>
<output>38</output>

<gadget id="calculator">20 + 38</gadget>
<output>58</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">58 * 5</gadget>
<output>290</output>

<result>290</result>