Uncle Zhang went to buy beer. One empty beer bottle can be refunded 0.5 yuan, and one bottle of beer is 2.9 yuan. Uncle Zhang refunded 5 empty beer bottles and bought 7 bottles of beer. How much money do you need to add?
张叔叔去买啤酒，一个空啤酒瓶可以退0.5元，单买一瓶啤酒2.9元，张叔叔退了5个空啤酒瓶，买了7瓶啤酒，需要添多少钱？
<gadget id="calculator">2.9 * 7</gadget>
<output>20.3</output>

<gadget id="calculator">5 * 0.5</gadget>
<output>2.5</output>

<gadget id="calculator">20.3 - 2.5</gadget>
<output>17.8</output>

<result>17.8</result>