A class lasts 40 minutes. The teacher and the students spent (1/4) hour interacting, the group worked (1/3), and the rest of the time the students practiced. How many hours did the students practice in this class?
一堂课40分钟，老师和学生互动用了(1/4)小时，小组合作用了(1/3)，其余时间学生练习，这堂课学生练习时间是多少小时？
<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(2/3) * (1/3)</gadget>
<output>2/9 = around 0.222222</output>

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

<gadget id="calculator">(2/3) - (2/9) - (1/4)</gadget>
<output>7/36 = around 0.194444</output>

<result>7/36 = around 0.194444</result>