comic strip costs 7.80 yuan, which is 5.40 yuan cheaper than a story. How much does it cost to buy these two books?
一本连环画7.80元，比一本故事会便宜5.40元．买这两本书共需多少元钱？
<gadget id="calculator">7.8 + 5.4 + 7.8</gadget>
<output>21</output>

<result>21</result>