A copy of "Selected Fairy Tales" is 6.9 yuan, and a copy of "Use Your Brain" is 3.4 yuan. How much cheaper is "Use Your Brain" than "Selected Fairy Tales"?
一本《童话故事选》6.9元，一本《动脑筋》3.4元，《动脑筋》比《童话故事选》便宜多少元？
<gadget id="calculator">6.9 - 3.4</gadget>
<output>3.5</output>

<result>3.5</result>