After filling 100 grams of 80% brine, pour out 40 grams of salt water, then fill the cup with water, stir, then pour out 40 grams of salt water, and then fill it up with water. After repeating this three times, the cup What is the concentration of the salt water?
在装满100克浓度为80%的盐水中倒出40克盐水后，再用清水将杯加满，搅拌后再倒出40克盐水，然后再用清水加满，如此反复三次后，杯中盐水的浓度是多少？
<gadget id="calculator">100 - 40</gadget>
<output>60</output>

<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">(4/5) / 100</gadget>
<output>1/125 = around 0.008</output>

<gadget id="calculator">60 * (1/125)</gadget>
<output>12/25 = around 0.48</output>

<gadget id="calculator">(12/25) / 100</gadget>
<output>3/625 = around 0.0048</output>

<gadget id="calculator">60 * (3/625)</gadget>
<output>36/125 = around 0.288</output>

<gadget id="calculator">(36/125) / 100</gadget>
<output>9/3_125 = around 0.00288</output>

<gadget id="calculator">60 * (9/3_125)</gadget>
<output>108/625 = around 0.1728</output>

<result>108/625 = around 0.1728</result>