In a mathematics class, the teacher spends (1/5) hours lecturing, the group works for (2/15) hours, and the rest of the time for exercises, a class is 35 minutes, how many hours did the students spend doing exercises?
在一节数学课上，老师讲课用了(1/5)小时，小组合作用了(2/15)小时，其余时间做练习，一节课35分钟，学生做练习用了多少小时？
<gadget id="calculator">7 / 12</gadget>
<output>7/12 = around 0.583333</output>

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

<gadget id="calculator">2 / 15</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">(1/5) + (2/15)</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(7/12) - (1/3)</gadget>
<output>1/4 = around 0.25</output>

<result>1/4 = around 0.25</result>