Question 786378
Car A -- 57km/h - 7 hours

Distance = 399 km distance

Car A travels 57*2=114 km in 2 hours 

it is 114 km ahead when Car B starts

Car B rate = 72 km/h

catch up speed = 72-57=15 km/h

distance = 114 to catch up
t=d/r
t=114/15
t= 7.6 hours to catch up