Example 5. Three test tubes A, B, and C each contain 10 grams, 20 grams, and 30 grams of water. Pour 10 grams of a certain mass fraction of brine into tube A, mix and pour 10 grams into tube B, and after mixing, take 10 grams from tube B and pour it into tube C. Now the mass fraction of brine in tube C is 0.5%. What is the mass fraction of brine that was first poured into tube A?
例题5．甲、乙、丙3个试管中各盛有10克、20克、30克水．把某种质量分数的盐水10克倒入甲管中，混合后取10克倒入乙管中，再混合后从乙管中取出10克倒入丙管中．现在丙管中的盐水的质量分数为0.5%．最早倒入甲管中的盐水质量分数是多少？
<gadget id="calculator">30 + 10</gadget>
<output>40</output>

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

<gadget id="calculator">40 * 0.005</gadget>
<output>0.2</output>

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

<gadget id="calculator">30 / 10</gadget>
<output>3</output>

<gadget id="calculator">0.2 * 3</gadget>
<output>0.6</output>

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

<gadget id="calculator">20 / 10</gadget>
<output>2</output>

<gadget id="calculator">0.6 * 2</gadget>
<output>1.2</output>

<gadget id="calculator">1.2 / 10</gadget>
<output>0.12</output>

<result>0.12</result>