class lasts for 40 minutes. In class, the students spend (1/4) hour discussing, communicating and reporting, the teacher spends (1/5) hour explaining, and the rest of the time is used for exercises. How long did it take for the exercises?
一节课40分钟，在课堂上同学们讨论交流、汇报用去(1/4)小时，老师讲解用去(1/5)小时，其余时间用来做练习，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 / 4</gadget>
<output>1/4 = around 0.25</output>

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

<gadget id="calculator">(2/3) - (1/4) - (1/5)</gadget>
<output>13/60 = around 0.216667</output>

<result>13/60 = around 0.216667</result>