A "story book" is 6.8 yuan, and a brains book is 3.4 yuan. What is the formula for calculating how much more expensive a story book is than a brains book
一本"故事书"6.8元，一本动脑筋3.4元，计算故事书比动脑筋书贵多少钱的计算式子多少
<gadget id="calculator">6.8 - 3.4</gadget>
<output>3.4</output>

<result>3.4</result>