There are 4 kg of 15% brine, and 0.5 kg of salt is added to the brine. If the concentration of the brine is to be 10%, how many kg of water needs to be added?
现有15%的盐水4千克，往盐水中加入0.5千克的盐，若要使盐水浓度变为10%，需要加清水多少千克？
<gadget id="calculator">15 / 100</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">4 * (3/20)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">4 * (1/10)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">0.5 * (1/10)</gadget>
<output>0.05</output>

<gadget id="calculator">0.5 - (2/5) - 0.05</gadget>
<output>0.05</output>

<gadget id="calculator">(3/5) + 0.05</gadget>
<output>0.65</output>

<gadget id="calculator">0.65 / (1/10)</gadget>
<output>6.5</output>

<result>6.5</result>