For a certain project, Xiao Wang completed it in 20 days alone, and Xiao Zhang completed it in 30 days alone. Now the two are working together, but Xiao Wang rested for 4 days in the middle, and Xiao Zhang also rested for several days. Finally, the project took 16 days to complete. How many days did Xiao Zhang rest?
某项工程，小王单独20天完成，小张单独30天完成．现在两人合作，但中间小王休息了4天，小张也休息了若干天，最后该工程用了16天完成．小张休息了多少天？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">16 - 4</gadget>
<output>12</output>

<gadget id="calculator">(1/20) * 12</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1 - (3/5)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(2/5) / (1/30)</gadget>
<output>12</output>

<gadget id="calculator">16 - 12</gadget>
<output>4</output>

<result>4</result>