After drinking 20% of a glass of milk, add water and stir well, then drink 50%, at this time, how much pure milk accounts for the capacity of the glass.
一杯牛奶喝去20%后加水搅匀，再喝去50%，这时纯牛奶占杯子容量的多少．
<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">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">(1/2) * (4/5)</gadget>
<output>2/5 = around 0.4</output>

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