There are 6 students in the first row, and there are 3 more students in each row than the previous row, making a total of 4 rows. How many students are there in total?
学生排队，第一排有6个同学，以后每一排比前一排多3人，共排了4排．一共有多少个同学？
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">3 * 3</gadget>
<output>9</output>

<gadget id="calculator">6 + 9</gadget>
<output>15</output>

<gadget id="calculator">6 + 15</gadget>
<output>21</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">21 * 2</gadget>
<output>42</output>

<result>42</result>