Sixth grade students have three choices for lunch: school cafeteria (35%), bring their own lunch (40%), and go home (25%). It is known that the number of students who bring their own lunch is 20 more than those who eat in the school cafeteria. How many students are there in the sixth grade?
六年级同学吃午餐有三种选择：学校食堂(35%)，自带午餐(40%)，回家(25%)，已知自己带午餐的学生人数比在学校食堂吃的多20人．问六年级一共有学生多少人？
<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">35 / 100</gadget>
<output>7/20 = around 0.35</output>

<gadget id="calculator">(2/5) - (7/20)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">20 / (1/20)</gadget>
<output>400</output>

<result>400</result>