There is some 40% salt water in the barrel. When 5 kilograms of water are added, the concentration is reduced to 30%. How many kilograms of salt are added to increase the concentration of salt water to 50%.
桶种有些40%的某种盐水，当加入5千克水后，浓度降低到30%，再加入多少千克盐，可使盐水的浓度提高到50%．
<gadget id="calculator">10 - 6</gadget>
<output>4</output>

<gadget id="calculator">4 / 0.5</gadget>
<output>8</output>

<result>8</result>