Teacher Zhang bought two reference books. "Detailed Explanation of Mathematics Teaching" is 19.56 yuan, and "Happy Math" is 2.5 yuan cheaper than "Detailed Explanation of Mathematics Teaching". How much does Mr. Zhang pay?
张老师买了两本参考书．《数学教学详解》19.56元，《快乐数学》比《数学教学详解》便宜2.5元．张老师应付多少钱？
<gadget id="calculator">19.56 - 2.5</gadget>
<output>17.06</output>

<gadget id="calculator">17.06 + 19.56</gadget>
<output>36.62</output>

<result>36.62</result>