A class lasts 40 minutes. The students spend (1/4) hour discussing and communicating with each other, the teacher spends (1/5) hour explaining, and the rest of the time the students practice independently. How many hours did the students practice independently?
一节课40分钟，学生互相讨论交流用了(1/4)小时，老师讲解用了(1/5)小时，其余时间学生独立练习，学生独立练习用了多少小时？
<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">(1/4) + (1/5)</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">(2/3) - (9/20)</gadget>
<output>13/60 = around 0.216667</output>

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