Bill climbs a two-mile hill at an uphill speed of two mph, spends no time at the top, and immediately walks down at six mph.What is his average speed for the up and down trips?

• 比尔爬一座两英哩的小山,他以每小时两英哩速度上山,没花任何时间在山顶上,就迅速地以每小时六英哩的速度下山,请问他上下山平均速度是每小时几英哩?