Teacher Wang bought two reference books. "Mathematics Class Homework" is 12.56 yuan each, and "Voice Calculation Daily Practice" is 3.5 yuan cheaper than "Mathematics Class Homework". How much does Mr. Wang pay?
王老师买了两本参考．《数学课时作业》每本12.56元，《口算速算天天练》比《数学课时作业》便宜3.5元．王老师应付多少元？
<gadget id="calculator">12.56 - 3.5</gadget>
<output>9.06</output>

<gadget id="calculator">12.56 + 9.06</gadget>
<output>21.62</output>

<result>21.62</result>