Zhang Jun and Liu Wei from Class 41 want to buy a football together. Zhang Jun has 13.2 yuan, Liu Wei has 16.8 yuan, and the price of a football is twice the sum of their money. How much money do they need to buy a football?
四一班的张军和刘伟两个同学要合买一个足球．张军有13.2元，刘伟有16.8元，一个足球的价钱是两人钱数总和的2倍，他们要买一个足球还差多少元？
<gadget id="calculator">13.2 + 16.8</gadget>
<output>30</output>

<result>30</result>