Some 4% salt water turns into 10% salt water after evaporating 30 kilograms of water. How many kilograms of salt are in the original brine?
有若干4%的盐水，蒸发掉30千克水后变成10%的盐水．问原来盐水中含盐多少千克？
<gadget id="calculator">0.1 - 0.04</gadget>
<output>0.06</output>

<gadget id="calculator">3 / 0.06</gadget>
<output>50</output>

<gadget id="calculator">4 / 100</gadget>
<output>1/25 = around 0.04</output>

<gadget id="calculator">50 * (1/25)</gadget>
<output>2</output>

<result>2</result>