After pouring 40 grams of salt water from a cup filled with 200 grams of 50% salt water, pour in clean water to fill up the cup. Stir well, then pour out 40 grams of salt water, and then pour in water to fill up the cup. After repeating this three times, what is the concentration of salt water in the cup?
从装满200克浓度为50%的盐水的杯中倒出40克盐水后，再倒入清水将杯加满．搅匀后再倒出40克盐水，然后再倒入清水将杯加满．这样反复三次后，杯中盐水的浓度是多少．
<gadget id="calculator">200 - 40</gadget>
<output>160</output>

<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(1/2) / 200</gadget>
<output>1/400 = around 0.0025</output>

<gadget id="calculator">160 * (1/400)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(2/5) / 200</gadget>
<output>1/500 = around 0.002</output>

<gadget id="calculator">160 * (1/500)</gadget>
<output>8/25 = around 0.32</output>

<gadget id="calculator">(8/25) / 200</gadget>
<output>1/625 = around 0.0016</output>

<gadget id="calculator">160 * (1/625)</gadget>
<output>32/125 = around 0.256</output>

<result>32/125 = around 0.256</result>