project was completed in 75 days by A alone, and 50 days by B alone. In the process of cooperation, A left some days in the middle, and the whole project was completed in 40 days. How many days did A leave?
一项工程，甲独做75天完成，乙独做50天完成，在合做过程中，甲中途离开了一些天数，结果整个工程40天才完成．甲中途离开了几天？
<gadget id="calculator">1 / 50</gadget>
<output>1/50 = around 0.02</output>

<gadget id="calculator">(1/50) * 40</gadget>
<output>4/5 = around 0.8</output>

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

<gadget id="calculator">1 / 75</gadget>
<output>1/75 = around 0.013333</output>

<gadget id="calculator">(1/5) / (1/75)</gadget>
<output>15</output>

<gadget id="calculator">40 - 15</gadget>
<output>25</output>

<result>25</result>