Naughty went to Xinhua Bookstore to buy books. I spent 12.3 yuan on technology books and 15.8 yuan on story books. How much did Naughty spend on buying the two books? He paid 30 yuan, should he get Ming Shaoyuan back?
淘气去新华书店买书，买科技书用了12.3元，买故事书用了15.8元．淘气买两种书共用了多少元？他付出了30元，应找回名少元？
<gadget id="calculator">12.3 + 15.8</gadget>
<output>28.1</output>

<gadget id="calculator">30 - 28.1</gadget>
<output>1.9</output>

<result>1.9</result>