There is salt water with a concentration of 15% in a container. If 20 kg of water is added, the concentration of salt water will become 10%. How many kg of salt is originally contained in this container?
一容器内有浓度为15%的盐水，若再加入20千克的水，则盐水的浓度变为10%，问这个容器内原来含有盐多少千克？
<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

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

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

<gadget id="calculator">15 / 100</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">1 - (3/20)</gadget>
<output>17/20 = around 0.85</output>

<gadget id="calculator">(17/20) / (3/20)</gadget>
<output>17/3 = around 5.666667</output>

<gadget id="calculator">9 - (17/3)</gadget>
<output>10/3 = around 3.333333</output>

<gadget id="calculator">20 / (10/3)</gadget>
<output>6</output>

<result>6</result>