The mall sells a drink, 4 yuan for every 5 bottles when it is purchased, and 5 yuan for every 4 bottles when it is sold. If you want to earn 180 yuan. So how many bottles do you need to sell?
商场卖一饮料，进货时每5瓶4元，售出时每4瓶5元，要想赚180元．那么需要卖出多少瓶．
<gadget id="calculator">5 / 4</gadget>
<output>5/4 = around 1.25</output>

<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">(5/4) - (4/5)</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">180 / (9/20)</gadget>
<output>400</output>

<result>400</result>