There are several liters of 25% alcohol solution, if 20 liters of water are added, then the concentration of the alcohol solution becomes 15%. How many liters of pure alcohol are in the original alcoholic solution?
有浓度为25%的酒精溶液若干升，若再加入20升水，那么酒精溶液浓度变为15%．原来酒精溶液中有纯酒精多少升．
<gadget id="calculator">25 / 100</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">15 / 100</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">(1/4) - (3/20)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">3 / (1/10)</gadget>
<output>30</output>

<gadget id="calculator">30 * (1/4)</gadget>
<output>15/2 = around 7.5</output>

<result>15/2 = around 7.5</result>