We know that Eli as already traveled 4 km when Philip starts

And Eli has taken 4/3 hrs to do this

So....let the time it takes Philip to reach the halway point = T (in hrs)

So....the distance Philip travels = 7T

And the distance that Eli travels = the 4 km he has traveled plus his rate times the time it takes Philip to reach the halfway point = 4 km + 3T

So....the distance that Eli travels = the distance that Philip travels ...so we have

4 + 3T = 7T

4 = 4T

T = 1 hr

So....Philip travels 7(1) = 7 km

And Eli travels 4 + 3(1) = 7 km

So...each travels 7km

It takes Eli (4/3)hrs + 1 hr = 7/3 hrs to travel halfway

And it takes Philip just 1 hour to travel halfway