Let the speed of train C be xkm∕h. At 9 p.m. the train A will have covered a distance of 180km. For trains A and B relative speed =(90−60)=30km∕h Distance between them =180km Time after which they meet =
180
30
=6hrs
For trains A and C relative speeds =(60+x)km∕h Distance between them =1080km.