A boat departs from point A and travels along the water at a speed of 60 km/h, arrives at point B in 2 hours, and travels against the current in 4 hours when returning. What is the speed of the boat when returning?
一艘船从甲地出发，顺水而行，船的速度是60千米/时，2小时到达乙地，返回时逆水而行，用了4小时．返回时船的速度是多少？
<gadget id="calculator">2 / 4</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">60 * (1/2)</gadget>
<output>30</output>

<result>30</result>