Li Feng wants to buy a set of stereos. If he pays within 30 days, he can get a 15% discount. If he pays within 10 days, he can get a 20% discount. He pays 4,000 yuan immediately after hearing it. What is the original price?
李峰想买一套音响，如果在30天内付款，可优惠15%，如果在10天内付款，可优惠20%，他听后立即付了4000元，那么原价是多少．
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">4_000 / (4/5)</gadget>
<output>5_000</output>

<result>5_000</result>