container contains 20 liters of pure alcohol. After pouring out 5 liters, add 5 liters of water. After pouring out 10 liters, add 10 liters of water. At this time, if the alcohol solution in the container is made into an alcohol solution with a concentration of 7.5%, how many liters of water should be added?
一容器内装有20升纯酒精，倒出5升后，又加入5升水，再倒出10升后，又加入10升水．此时若把容器内的酒精溶液配制成浓度为7.5%的酒精溶液，还要加多少升水？
<gadget id="calculator">20 - 5</gadget>
<output>15</output>

<gadget id="calculator">15 / 20</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">(3/4) * 10</gadget>
<output>15/2 = around 7.5</output>

<gadget id="calculator">20 - 5 - (15/2)</gadget>
<output>15/2 = around 7.5</output>

<gadget id="calculator">7.5 / 100</gadget>
<output>0.075</output>

<gadget id="calculator">(15/2) / 0.075</gadget>
<output>100</output>

<gadget id="calculator">100 - 20</gadget>
<output>80</output>

<result>80</result>