For a manuscript, two typists A and B need 10 copies to complete it. After 4 days of typing together, the rest of the manuscript will be typed by B alone, and it will take 21 days to complete. If A alone typed the manuscript How many days will it take? (column solution)
一部书稿，甲，乙两个打字员合打需要10开可以完成，两人合打了4天后，余下的书稿由乙单独打，还要21天才能完成，这部书稿如果由甲单独打需要多少天？(列式解答)
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/10) * 4</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">(3/5) / 21</gadget>
<output>1/35 = around 0.028571</output>

<gadget id="calculator">(1/10) - (1/35)</gadget>
<output>1/14 = around 0.071429</output>

<gadget id="calculator">1 / (1/14)</gadget>
<output>14</output>

<result>14</result>