Xiaomei buys a pen and a box of watercolor pens for 8 yuan. Xiaohua buys the same pen and a storybook for 15 yuan. Which is more expensive, a box of watercolor pens or a story book? How much is it expensive?
小美买一支钢笔和一盒水彩笔共用去8元．小花买一支同样的钢笔和一本故事书共用去15元钱．一盒水彩笔和一本故事书哪个贵？贵多少元？
<gadget id="calculator">15 - 8</gadget>
<output>7</output>

<result>7</result>