The master and apprentice cooperate to process a batch of parts in 15 days, and the master stops work for 4 days due to some accidents, so it takes 18 days to complete the task. How many days will it take for the apprentice to complete this task alone?
师傅和徒弟合作加工一批零件15天完成，中途师傅因事停做4天，这样完成任务共用18天．如徒弟单独完成这项任务要多少天？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">18 - 4</gadget>
<output>14</output>

<gadget id="calculator">(1/15) * 14</gadget>
<output>14/15 = around 0.933333</output>

<gadget id="calculator">1 - (14/15)</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/15) / 18</gadget>
<output>1/270 = around 0.003704</output>

<gadget id="calculator">1 / (1/270)</gadget>
<output>270</output>

<result>270</result>