Fill a glass with pure alcohol, pour out 10% and fill it up with water, then pour out 10% and fill it up with water, what is the alcohol concentration at this time?
一个杯子中装满纯酒精，倒出10%用水加满，再倒出10%用水加满，这时的酒精浓度是多少．
<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">10 * (1/10)</gadget>
<output>1</output>

<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<gadget id="calculator">9 / 10</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">9 - (9/10)</gadget>
<output>81/10 = around 8.1</output>

<gadget id="calculator">(81/10) / 10</gadget>
<output>81/100 = around 0.81</output>

<gadget id="calculator">100 / 100</gadget>
<output>1</output>

<gadget id="calculator">(81/100) * 1</gadget>
<output>81/100 = around 0.81</output>

<result>81/100 = around 0.81</result>