An air conditioner is sold at a 10% discount, and the customer can pay 400 yuan less. How much is the original price of this air conditioner?
一台空调打九折出售，顾客可少付400元，这台空调原来售价多少元？
<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">1 - (9/10)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">400 / (1/10)</gadget>
<output>4_000</output>

<result>4_000</result>