There are three sulfuric acid solutions A, B and C. If A and B are mixed in a mass ratio of 3:4, sulfuric acid with a concentration of 17.5% is obtained; if A and B are mixed in a mass ratio of 2:5, sulfuric acid with a concentration of 14.5% is obtained; if A, B, C is mixed according to the mass ratio of 5:9:10, and sulfuric acid with a concentration of 21% can be obtained, and the concentration of C solution is requested.
现有甲、乙、丙三种硫酸溶液．如果把甲、乙按照3：4的质量比混合，得到浓度为17.5%的硫酸；如果把甲、乙按照2：5的质量比混合，得到浓度为14.5%的硫酸；如果把甲、乙、丙按照5：9：10的质量比混合，可以得到浓度为21%的硫酸，请求出丙溶液的浓度．
<gadget id="calculator">5 + 9 + 10</gadget>
<output>24</output>

<gadget id="calculator">21 / 100</gadget>
<output>21/100 = around 0.21</output>

<gadget id="calculator">24 * (21/100)</gadget>
<output>126/25 = around 5.04</output>

<gadget id="calculator">14.5 / 100</gadget>
<output>0.145</output>

<gadget id="calculator">17.5 / 100</gadget>
<output>0.175</output>

<gadget id="calculator">0.175 - 0.145</gadget>
<output>0.03</output>

<gadget id="calculator">0.03 * 2</gadget>
<output>0.06</output>

<gadget id="calculator">0.145 - 0.06</gadget>
<output>0.085</output>

<gadget id="calculator">0.085 * 9</gadget>
<output>0.765</output>

<gadget id="calculator">0.03 * 4</gadget>
<output>0.12</output>

<gadget id="calculator">0.175 + 0.12</gadget>
<output>0.295</output>

<gadget id="calculator">0.295 * 5</gadget>
<output>1.475</output>

<gadget id="calculator">(126/25) - 0.765 - 1.475</gadget>
<output>2.8</output>

<gadget id="calculator">2.8 / 10</gadget>
<output>0.28</output>

<result>0.28</result>