Hi Hapa,
Recall that 'Distance = Rate X Time'
d = rt
t =d/r
Let p = rate of prop plane
Let 2p = rate of jet plane
Let = time for the prop plane
Let = time for the jet plane
The distance is constant, and since the prop plane is 3 hours behind the jet plane, the two times wll equate this way:
Multiply through by 2p
3000 = 1500 +6p
6p = 1500
p = 250 mph for the prop plane
The jet travels 500 mph