Li Jia saved 19.4 yuan in pocket money in April. He wants to buy 4 books for the children in the disaster area, each of which is 7.8 yuan. How much is the difference?
李佳四月份节省零用钱19.4元，他想为灾区小朋友买4本图书，每本7.8元，还差多少元？
<gadget id="calculator">7.8 * 4</gadget>
<output>31.2</output>

<gadget id="calculator">31.2 - 19.4</gadget>
<output>11.8</output>

<result>11.8</result>