Question 1186092
4/4=1km
6/4=1.5km per 1km of his (he is .5km faster to go the same 1km as her or basically she goes 2/3's his speed) 
9 total km / 1.5 km= 6
9-6=3
so he needs to give her a 3km head start

so for every 2km she runs he is on the 3rd km
so when she is at 4km he's at 6km
so when she is at 6km he's at 9km
that's a 3km gap which is why he needs to give her a 3km head start.