5pm

In one hour Alice has gone 2.5 miles and Sue has gone 5 miles

In two hours Alice has gone 5 miles

Alice took one hour longer than Sue to get 5 miles

Alice's speed was 1/2 as fast as Sue's speed.

They started at 4pm. One hour later Sue arrived, at 5pm

Sometimes just trying some simple numbers gets you the answer faster,

and if not at least narrows down what general range the answer should be in.