There are three bottles of solutions A, B, and C, the concentrations of which are 63%, 42%, and 28% in turn, and C has 30 liters. When A and B are mixed, the concentration becomes 49%. At this time, we mix the mixed solution with C to obtain a concentration of 42% solution, so how many liters is the original A?
有甲、乙、丙三瓶溶液，浓度依次为63%、42%、28%，丙有30升，当甲乙混和后，浓度变为49%，这时我们把混合液再与丙混和得到浓度为42%的溶液，那么原来甲有多少升．
<gadget id="calculator">42 / 100</gadget>
<output>21/50 = around 0.42</output>

<gadget id="calculator">28 / 100</gadget>
<output>7/25 = around 0.28</output>

<gadget id="calculator">(21/50) - (7/25)</gadget>
<output>7/50 = around 0.14</output>

<gadget id="calculator">49 / 100</gadget>
<output>49/100 = around 0.49</output>

<gadget id="calculator">(49/100) - (21/50)</gadget>
<output>7/100 = around 0.07</output>

<gadget id="calculator">(7/50) / (7/100)</gadget>
<output>2</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">30 / (1/2)</gadget>
<output>60</output>

<gadget id="calculator">1 + 2</gadget>
<output>3</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">60 * (1/3)</gadget>
<output>20</output>

<result>20</result>