bottle of pure alcohol weighs 200 grams, 20 grams are used up, and then filled with water, what is the concentration of alcohol at this time
一瓶纯酒精重200克，用掉了20克，再用水填满，这时酒精的浓度为多少
<gadget id="calculator">200 - 20</gadget>
<output>180</output>

<gadget id="calculator">180 / 200</gadget>
<output>9/10 = around 0.9</output>

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

<gadget id="calculator">(9/10) * 1</gadget>
<output>9/10 = around 0.9</output>

<result>9/10 = around 0.9</result>