A copy of "Children's Literature and Art" is priced at 4.2 yuan, and a copy of "Science and Technology Expo" is priced at 6.8 yuan. Wang Fang took 20 yuan to buy these two books, how much should she get back?
一本《儿童文艺》的售价是4.2元，一本《科技博览》售价6.8元．王芳拿20元去买这两本书，应找回多少元？
<gadget id="calculator">4.2 + 6.8</gadget>
<output>11</output>

<gadget id="calculator">20 - 11</gadget>
<output>9</output>

<result>9</result>