From his home in Banff, Dan drove to Jasper at an average speed of 75 km/h. From her home in Canmore, Ashley drove through Banff to Jasper at an average speed of 85 km/h. The distance from Canmore to Banff is 18 km. If Dan and Ashley left home at the same time,
a) after what length of time did Ashley overtake Dan?
b) how far were they from Banff when Ashley overtook Dan?