There is a shampoo in the supermarket with an original price of 20 yuan. Now there is a promotion for buying four bottles and getting one free. If you buy four bottles at a time, how much is each bottle cheaper than the original price?
超市有一种洗发液原价20元，现在搞促销买四瓶送一瓶，如果一次买四瓶，每瓶比原价便宜了多少钱？
<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>