After mixing 3 kg of 20% salt water with 1 kg of 30% salt water, what is the concentration of the salt water?
把3公斤20%的盐水与1公斤30%的盐水混合后，求盐水的浓度是百分之几．
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">3 * (1/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 * (3/10)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(3/5) + (3/10)</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">(9/10) / 4</gadget>
<output>9/40 = around 0.225</output>

<result>9/40 = around 0.225</result>