A container contains sulfuric acid with a concentration of 45%. If 16 kg of water is added, the concentration becomes 25%. How many kg of pure sulfuric acid is contained in this container?
一容器内盛有浓度为45%的硫酸，若再加入16千克的水，则浓度变为25%，这个容器内原来含有纯硫酸多少千克？
<gadget id="calculator">25 / 100</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/4) * 16</gadget>
<output>4</output>

<gadget id="calculator">45 / 100</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">(9/20) - (1/4)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">4 / (1/5)</gadget>
<output>20</output>

<gadget id="calculator">20 * (9/20)</gadget>
<output>9</output>

<result>9</result>