For a batch of parts, the workers in class A worked for 7 hours to complete half of the task. Afterwards, the workers in class A and B worked together, and it took another 4 hours to finish all the parts. If this batch of parts was made by workers in class B, how much time would it take?
一批零件，甲班工人工作7小时完成了任务的一半，以后甲乙班工人一起做，又经过4小时才全部做完，这批零件如果让乙班工人做，要多少时间？
<gadget id="calculator">3 / 64</gadget>
<output>3/64 = around 0.046875</output>

<gadget id="calculator">1 / (3/64)</gadget>
<output>64/3 = around 21.333333</output>

<result>64/3 = around 21.333333</result>