A professional chicken farmer raised some chickens, and after selling 20% of them, there were 1280 chickens left. How many chickens did this professional chicken farmer have?
一个养鸡专业户养了一些鸡，卖出20%后还剩1280只．这个养鸡专业户原来有鸡多少只？
<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">1_280 / (4/5)</gadget>
<output>1_600</output>

<result>1_600</result>