A job alone takes 7 hours, and B alone takes 12 hours. When A and B complete the job together, B will do much less than A.
一件工作，甲独做要7小时，乙独做要12小时，甲乙合做完成时，乙比甲少做多少．
<gadget id="calculator">7 + 12</gadget>
<output>19</output>

<gadget id="calculator">12 / 19</gadget>
<output>12/19 = around 0.631579</output>

<gadget id="calculator">7 / 19</gadget>
<output>7/19 = around 0.368421</output>

<gadget id="calculator">(12/19) - (7/19)</gadget>
<output>5/19 = around 0.263158</output>

<result>5/19 = around 0.263158</result>