Three test tubes A, B, and C each contain 10 grams, 20 grams, and 30 grams of water. Pour 10 grams of salt water of a certain concentration into A, take out 10 grams after mixing and pour it into B, take out 10 grams from B and pour it into C after mixing. Now the concentration of brine in C is 0.5%. What is the concentration of the brine that was first poured into A?
A，B，C三个试管中各盛有10克、20克、30克水．把某种浓度的盐水10克倒入A中，混合后取出10克倒入B中，混合后又从B中取出10克倒入C中．现在C中盐水浓度是0.5%．问最早倒入A中的盐水浓度是多少？
<gadget id="calculator">10 + 10</gadget>
<output>20</output>

<gadget id="calculator">20 + 10</gadget>
<output>30</output>

<gadget id="calculator">30 + 10</gadget>
<output>40</output>

<gadget id="calculator">0.5 / 100</gadget>
<output>0.005</output>

<gadget id="calculator">0.005 / 10</gadget>
<output>0.0005</output>

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

<gadget id="calculator">40 * 0.0005 * 1</gadget>
<output>0.02</output>

<gadget id="calculator">0.02 / 10</gadget>
<output>0.002</output>

<gadget id="calculator">30 * 0.002 * 1</gadget>
<output>0.06</output>

<gadget id="calculator">0.06 / 10</gadget>
<output>0.006</output>

<gadget id="calculator">20 * 0.006</gadget>
<output>0.12</output>

<result>0.12</result>