There are 15 rows of seats in a multimedia classroom in a certain school. The first row has 20 seats, and each row has 2 more seats than the front row. How many seats are there in this classroom?
某校多媒体教室，共有15排座位，第一排共有20个座位，以后每排都比前排多2个座位，这个教室共有多少个座位？
<gadget id="calculator">15 - 1</gadget>
<output>14</output>

<gadget id="calculator">14 * 2</gadget>
<output>28</output>

<gadget id="calculator">20 + 28</gadget>
<output>48</output>

<gadget id="calculator">20 + 48</gadget>
<output>68</output>

<gadget id="calculator">15 / 2</gadget>
<output>15/2 = around 7.5</output>

<gadget id="calculator">68 * (15/2)</gadget>
<output>510</output>

<result>510</result>