It's all about...rate
A man "A" set out from a certain point and traveled at the rate of 6 kph. After "A" had gone for two hours, another man "B" set out to overtake "A" and went 4 km the first hour, 5 km the second hour, 6 km the third hour and so on gaining 1 km every hour. How many hours after "B"left were they together?