After drinking 20% of a glass of pure milk, fill it up with water and stir well, then drink 50%, then fill it up with water and mix well, then drink 30%, then the pure milk in the cup accounts for 100% of the volume of the cup Several?
一杯纯牛奶，喝去20%以后，用水加满搅匀，再喝去50%，然后用水加满搅匀，接着再喝去30%，这时杯子中的纯牛奶占杯子容积的百分之几？
<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">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 - (3/10)</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">(7/10) / 1</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">(4/5) * (1/2) * (7/10)</gadget>
<output>7/25 = around 0.28</output>

<result>7/25 = around 0.28</result>