After drinking 30% of a glass of pure milk, fill it up with water, stir well and then drink 50%, at this time, how much pure milk in the cup accounts for the capacity of the cup.
一杯纯牛奶，喝去30%后，加满水，搅匀后再喝去50%，这时杯中的纯牛奶占杯子容量的多少．
<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">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">(7/10) * (1/2)</gadget>
<output>7/20 = around 0.35</output>

<result>7/20 = around 0.35</result>