It takes 20 days for the master to process a batch of parts, and 25 days for the apprentice to do it alone. After 4 days of joint processing, the master leaves for something, and the remaining parts are completed by the apprentice alone. How many days will it take?
加工一批零件，师傅单独做需要20天，徒弟单独做需要25天，师徒共同加工4天后，师傅有事离开，剩下的零件由徒弟单独完成，还要几天完成？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 25</gadget>
<output>1/25 = around 0.04</output>

<gadget id="calculator">(1/20) + (1/25)</gadget>
<output>9/100 = around 0.09</output>

<gadget id="calculator">(9/100) * 4</gadget>
<output>9/25 = around 0.36</output>

<gadget id="calculator">1 - (9/25)</gadget>
<output>16/25 = around 0.64</output>

<gadget id="calculator">(16/25) / (1/25)</gadget>
<output>16</output>

<result>16</result>