In order to answer this questions we need to calculate the Speed of the object.
In this case, distance between Jill and Joe=d=30km
Speed of Jill=2km/h
Time it takes for Jill and Joe to meet = t = 3h
Distance covered by Jill= 2km/h * 3h =6km
Distance covered by Joe= d-6km= 30km-6km=24km
Speed of Joe = 24km/t=24km/3h=8km/h
Joe is walking at 8km/h