After pouring out 50 grams of salt water from 100 grams of 20% salt water, fill the cup with water, stir, then pour out 50 grams of salt water, and then fill the cup with water. Repeat this three times, what is the concentration of salt water in the cup?
从装满100克20%的盐水中倒出50克盐水后，在用清水将杯加满，搅拌后再倒出50克盐水，然后再用清水将杯加满．如此反复三次，杯中盐水的浓度是多少
<gadget id="calculator">100 - 50</gadget>
<output>50</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/5) / 100</gadget>
<output>1/500 = around 0.002</output>

<gadget id="calculator">50 * (1/500)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/10) / 100</gadget>
<output>1/1_000 = around 0.001</output>

<gadget id="calculator">50 * (1/1_000)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/20) / 100</gadget>
<output>1/2_000 = around 0.0005</output>

<gadget id="calculator">50 * (1/2_000)</gadget>
<output>1/40 = around 0.025</output>

<result>1/40 = around 0.025</result>