certain bookstore launched a discount card in the promotional activities. Each card was launched by a certain bookstore in the promotional activities. The price of each card is 20 yuan. You can enjoy a 20% discount when you buy books with the card. When buying books in the bookstore, when he checks out, he first buys a discount card and pays 180 yuan with the card. How much can Xiao Ming save by buying books this time?
某书店在促销活动中，推出一种优惠卡，每张卡某书店在促销活动中，推出一种优惠卡，每张卡售价二十元，凭卡购买书可享受8折优惠，小明到该书店购书，结账时，他先买优惠卡，凭卡付款180元，小明此次购书能省多少元．
<gadget id="calculator">180 - 20</gadget>
<output>160</output>

<gadget id="calculator">160 / 0.8</gadget>
<output>200</output>

<gadget id="calculator">200 - 180</gadget>
<output>20</output>

<result>20</result>