Aunt Wang's car currently has 24 liters of gasoline, and each liter of gasoline can travel 6.8km. Aunt Wang is going to travel by car. If the round-trip distance is 306km, how many liters of gasoline does Aunt Wang need to add at least?
王阿姨的车现有24升汽油，每升汽油可行驶6.8km．王阿姨准备开车外出旅游，如果往返的路程是306km，王阿姨至少还需加多少升汽油？
<gadget id="calculator">306 / 6.8</gadget>
<output>45</output>

<gadget id="calculator">45 - 24</gadget>
<output>21</output>

<result>21</result>