In a 40-minute math class, the teacher spent (3/20) hours explaining, the students spent (1/4) hours discussing and communicating, and the rest of the time students practiced. How many hours did the students practice?
一节数学课40分钟，老师讲解用了(3/20)小时，学生讨论交流用了(1/4)小时，其余的时间学生练习，学生练习用了多少小时？
<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">3 / 20</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(3/20) + (1/4)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(2/3) - (2/5)</gadget>
<output>4/15 = around 0.266667</output>

<result>4/15 = around 0.266667</result>