A, B, and C all make 20 of the same parts, and start to make them at the same time. When A finishes, B makes 15, and C makes 12. When B finishes, how many parts does C have left?
甲乙丙三个人都做同样的零件20个，同时开始做，甲做完时，乙做了15个，丙做了12个，乙做完时丙还有多少个没做．
<gadget id="calculator">12 / 15</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">20 * (4/5)</gadget>
<output>16</output>

<gadget id="calculator">20 - 16</gadget>
<output>4</output>

<result>4</result>