How about this way of thinking about it:
At 2.10 the first car has been going 1/6 of an hour so has travelled 15km (90/6).
The second car is then closing the gap at the difference between their speeds i.e. 10km/h.
So to close a 15km gap at a rate of 10km/h takes an hour and a half.
Add the ten minutes the second car was waiting you get the 1h40m previously quoted.
So the answer is 3.40