The shopping mall held a promotion, 20 yuan per bag of washing powder, buy 4 bags and get 1 bag free, mom bought 4 bags, how much is each bag cheaper?
商场搞了一次促销活动，每袋洗衣粉20元，买4袋送一袋，妈妈买了4袋，每袋便宜多少元？
<gadget id="calculator">4 + 1</gadget>
<output>5</output>

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

<gadget id="calculator">20 * (4/5)</gadget>
<output>16</output>

<gadget id="calculator">20 - 16</gadget>
<output>4</output>

<result>4</result>