Add a certain amount of water to an alcohol solution with a certain concentration, and the concentration becomes 30%, and then add the same amount of water, and the concentration becomes 20%, and then add the same amount of water, what is the concentration?
在有一点浓度的酒精溶液中加入一定量的水，浓度变为30%，再同样多的水，浓度变为20%，再加入同样多的水，浓度变为多少？
<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(3/10) - (1/5)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/5) - (3/10)</gadget>
<output>-1/10 = around -0.1</output>

<gadget id="calculator">(1/5) + (-1/10)</gadget>
<output>1/10 = around 0.1</output>

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

<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">1 * 3</gadget>
<output>3</output>

<gadget id="calculator">1 + 3</gadget>
<output>4</output>

<gadget id="calculator">(3/10) / 4</gadget>
<output>3/40 = around 0.075</output>

<gadget id="calculator">2 * (3/40)</gadget>
<output>3/20 = around 0.15</output>

<result>3/20 = around 0.15</result>