70% of the students in the class participated in the biology group, 75% of the students participated in the chemistry group, 85% of the students participated in the physics group, and 90% of the students participated in the mathematics group. Then what is the percentage of students who participate in all four groups at least in the whole class?
全班有70%的同学参加了生物小组，75%的同学参加了化学小组，85%的同学参加了物理小组，90%的同学参加了数学小组．则四个小组都去参加的同学至少占全班的百分比是多少．
<gadget id="calculator">75 - 100</gadget>
<output>-25</output>

<gadget id="calculator">70 + (-25)</gadget>
<output>45</output>

<gadget id="calculator">85 - 100</gadget>
<output>-15</output>

<gadget id="calculator">45 + (-15)</gadget>
<output>30</output>

<gadget id="calculator">90 - 100</gadget>
<output>-10</output>

<gadget id="calculator">30 + (-10)</gadget>
<output>20</output>

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

<gadget id="calculator">100 / 100</gadget>
<output>1</output>

<gadget id="calculator">(1/5) * 1</gadget>
<output>1/5 = around 0.2</output>

<result>1/5 = around 0.2</result>