Explanation: Speed, take a day trip for two days. Source: "Loss of Jie Ting": "Sima Yi back opposite direction and eighth has come to town, Mengda by surprise."
No. 2
成语发音： bèi dào ér xíng
成语释疑： 倍：加倍；道：行程。加快速度，一天走两天的行程。
成语出处： 《失街亭》：“司马懿背道而行，八日已到新城，孟达措手不及。”