Algebra 1 · Superchallenge problem

Car A leaves a town at 9:00 and drives at $60$ km/h. Car B leaves the same town at 9:30 and drives along the same road in the same direction at $80$ km/h. Car C leaves the same town at 10:00, also along that road, at a constant speed, and it passes cars A and B at the same moment. What is the speed of car C, in km/h?