In an experimental class, the teacher used (3/10) to explain the class, the students used (1/5) to do experiments, and (1/20) to communicate with students, and the rest of the time students did their homework. What percentage of the class time do students spend doing homework?
在一堂实验课上，老师讲解用了这堂课的(3/10)，学生做实验用了(1/5)，学生交流用了(1/20)，其余时间学生做作业．学生做作业所用的时间占这堂课的几分之几？
<gadget id="calculator">3 / 10</gadget>
<output>3/10 = around 0.3</output>

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

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 - (3/10) - (1/5) - (1/20)</gadget>
<output>9/20 = around 0.45</output>

<result>9/20 = around 0.45</result>