One project can be completed in 72 days by A alone, and 24 days by B alone. If A works for a certain number of days first, and then B completes the rest of the project alone, it will take a total of 36 days. Ask how many days did A do it first?
一件工程，甲独做72天完成，乙独做24天完成．若甲先做若干天后，由乙接着单独完成余下的工程，总共须要36天．问甲先做了多少天﹖
<gadget id="calculator">1 / 24</gadget>
<output>1/24 = around 0.041667</output>

<gadget id="calculator">36 * (1/24)</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">(3/2) - 1</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 72</gadget>
<output>1/72 = around 0.013889</output>

<gadget id="calculator">(1/24) - (1/72)</gadget>
<output>1/36 = around 0.027778</output>

<gadget id="calculator">(1/2) / (1/36)</gadget>
<output>18</output>

<result>18</result>