hi ! im having a difficult time answering this word problem.

"Alvin set out from a certain point and travelled at the rate of 6 kph. After Alvin had gone for two hours, Ben set on to overtake him and went 4km the first hour, 5 km the second hour, 6 km the third hour and so on, gaining 1 km every hour. After many hours were they together."

I keep on getting 5 using arithmetic progression. Could someone please please give me a hint how to solve this one ???

Thank you very much !!