The "buy 4 get 1 free" in the promotional activities is equivalent to selling at the original price.
促销活动中的"买4送1"相当于以原价的多少销售．
<gadget id="calculator">1 + 4</gadget>
<output>5</output>

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

<result>4/5 = around 0.8</result>