The fourth, fifth and sixth grades of Guangming Primary School held a calligraphy and painting competition. The students who won the first prize accounted for 20% of the number of winners, the ratio of the number of people who won the second prize to the third prize was 5:7, and there were 4 people who won the second prize than the third prize. How many students are there in total?
光明小学四、五、六年级举行书画比赛．获得一等奖的同学占获奖人数的20%，二等奖与三等奖的人数之比是5：7，获得二等奖的比三等奖少4人．获奖同学一共有多少人？
<gadget id="calculator">7 / 12</gadget>
<output>7/12 = around 0.583333</output>

<gadget id="calculator">5 / 12</gadget>
<output>5/12 = around 0.416667</output>

<gadget id="calculator">(7/12) - (5/12)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">4 / (1/6) / (4/5)</gadget>
<output>30</output>

<result>30</result>