A class lasts for 40 minutes, students use (1/5) hours for discussion, teachers use (3/10) hours for explanations, and the rest of the time is used for exercises. How long did it take you to do the exercises?
一节课有40分钟，学生讨论用(1/5)小时，教师讲解用(3/10)小时，剩下的时间用来做练习．做练习用了多长时间？
<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

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

<gadget id="calculator">3 / 10</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(2/3) - (1/5) - (3/10)</gadget>
<output>1/6 = around 0.166667</output>

<result>1/6 = around 0.166667</result>