B starts 1 hour ahead, so that equals 70 km ahead, agree? (70 km/h x 1h = 70 km)
So, the distance between B an A, when A starts is 70 km.
A is catching up to B at 20 km/h, right? (90 - 70 = 20)
So, it takes 70/20 or 3.5h for A to catch up to B.
Where do they meet? I'll leave that math for you!
Hope that helps!