A commodity is priced at 120% of the cost and then sold at a 10% discount. The result is a profit of 400 yuan. What is the cost of this commodity?
一种商品，按成本的120%定价后打九折出售，结果赚了400元，这种商品的成本是多少元．
<gadget id="calculator">120 / 100</gadget>
<output>6/5 = around 1.2</output>

<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">(6/5) * (9/10)</gadget>
<output>27/25 = around 1.08</output>

<gadget id="calculator">(27/25) - 1</gadget>
<output>2/25 = around 0.08</output>

<gadget id="calculator">400 / (2/25)</gadget>
<output>5_000</output>

<result>5_000</result>