A class lasts for 40 minutes, the students spend (1/5) hour on the experiment, the teacher spends (1/4) hour on the explanation, and the rest of the time the students do their homework, how many hours did they spend doing the homework?
一堂课40分钟，学生实验用了(1/5)小时，老师讲解用了(1/4)小时，其余的时间学生做作业，做作业用了多少小时？
<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">1 / 4</gadget>
<output>1/4 = around 0.25</output>

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

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