It takes 15 hours for an engineer to do it alone, and 18 hours for an apprentice to do it alone. Now the master will do the work for 5 hours first, and the apprentice will do the rest. How many hours will it take to finish?
一项工程师傅独做要15小时完成，徒弟独做要18小时完成．现在由师傅先做5小时，余下的由徒弟做，还要几小时才能完成？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

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

<gadget id="calculator">1 - (1/3)</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">1 / 18</gadget>
<output>1/18 = around 0.055556</output>

<gadget id="calculator">(2/3) / (1/18)</gadget>
<output>12</output>

<result>12</result>