The first time two cars met, they walked a whole journey, and the second time they met, they walked three whole journeys. When train B meets for the first time, it is 40km away from Station A, so the formula 40X3= 120 (km) is listed. Two cars, A and B, have done the whole journey three times, so we take train B as the benchmark (as can be seen from the figure), and train A has done the whole journey +40-20=20 (km) at the second meeting, so 120= one whole journey +20km.

Whole journey = 120-20= 100 (km)