Class 4.2 students went to climb the mountain, the speed is 420 m/h, and it took 4 hours to go up the mountain. It only took 2 hours to go down the mountain and return the same way. What is the speed of going down the mountain?
4.2班同学去爬山，上山速度是420米/时，上山用了4小时．下山按原路返回，却只用了2小时，下山速度是多少？
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">420 * 2</gadget>
<output>840</output>

<result>840</result>