A cup of pure milk, drink 20%, fill up with water and stir well, then drink 50%, and then fill up with water. At this time, how much pure milk in the cup accounts for the solution in the cup.
一杯纯牛奶，喝去了20%，加满水搅匀，再喝去50%，然后再加满水．这时杯中纯牛奶占杯中溶液的多少．
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

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

<gadget id="calculator">1 - (1/2)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(1/2) / 1</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(4/5) * (1/2)</gadget>
<output>2/5 = around 0.4</output>

<result>2/5 = around 0.4</result>