Xiaofang bought 3 books in the bookstore with 30 yuan, and the unit price of each book is a yuan; when a=7.8, how much yuan is left.
小方用30元钱到书店买了3本书，每本书的单价a元；当a=7.8时，还剩多少元．
<gadget id="calculator">3 * 7.8</gadget>
<output>23.4</output>

<gadget id="calculator">30 - 23.4</gadget>
<output>6.6</output>

<result>6.6</result>