There is 30kg of brine with 25% salt content, now add water, how much water needs to be added to reduce the content to 15%?
有含盐25%的盐水30千克，现加入清水，要使其含量降低为15%，需加清水多少？
<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>5/3 = around 1.666667</output>

<gadget id="calculator">30 * (5/3)</gadget>
<output>50</output>

<gadget id="calculator">50 - 30</gadget>
<output>20</output>

<result>20</result>