There are 30 liters of gasoline in the fuel tank of Uncle Li's car, and each liter of gasoline can travel 9.5km. Uncle Li took his apprentices to practice driving. He was driving at an average speed of 57km/h on the highway. How much time can he drive before he must drive back?
李叔叔汽车的油箱里有30升汽油，每升汽油可行驶9.5km．李叔叔带徒弟练习开车，在公路上以平均每时57km的速度行驶，最多只能开多少时就必须往回开？
<gadget id="calculator">30 * 9.5</gadget>
<output>285</output>

<gadget id="calculator">285 / 57 / 2</gadget>
<output>5/2 = around 2.5</output>

<result>5/2 = around 2.5</result>