A batch of parts is finished separately, A needs 20 days, and B needs 15 days. If A does it for 4 days first and then B does the rest, how many days will it take?
一批零件，单独做完，甲需20天，乙需15天．如果甲先做4天后剩下的由乙独做，还要多少天？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/20) * 4</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(4/5) / (1/15)</gadget>
<output>12</output>

<result>12</result>