The sixth grade students signed up to participate in the mathematics interest group, and the number of students who participated was 1/3 of the total number of sixth graders. Later, 20 people joined. At this time, the ratio of the number of students who participated to the number of people who did not participate was 3:4. How many people are there in the sixth grade?
六年级学生报名参加数学兴趣小组，参加的同学是六年级总人数的(1/3)，后来有20人参加，这时参加的同学与未参加的人数的比是3：4．六年级一共有多少人？
<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(3/7) - (1/3)</gadget>
<output>2/21 = around 0.095238</output>

<gadget id="calculator">20 / (2/21)</gadget>
<output>210</output>

<result>210</result>