There are several grams of salt water with a concentration of 20%. If 500 grams of salt are added, the concentration of the salt water will be 40% after it is completely dissolved. How many grams of the original salt solution is there?
有浓度为20%的盐水若干克，如果加入500克盐，完全溶解后盐水的浓度为40%，原来盐水溶液有多少克？
<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">500 * (2/5)</gadget>
<output>200</output>

<gadget id="calculator">500 - 200</gadget>
<output>300</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(2/5) - (1/5) - 1</gadget>
<output>-4/5 = around -0.8</output>

<gadget id="calculator">1 + (-4/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">300 / (1/5)</gadget>
<output>1_500</output>

<result>1_500</result>