The speed of the freight train: x
the speed of the passenger train is therefore x+15

The time it takes an object traveling with speed v to travel distance D is:

t = D/v

Hence the time it takes the freight train to travel 390 miles is:

t = 390/x

And the time it takes the passenger train to travel 480 miles is:

t= 480/(x+15)

Since the two times are the same we have:

390/x = 480/(x+15)

390(x+15) = 480x

39x+39*15 = 48x

585 = 9x

x = 65mph

The speed of the freight train is x=65mph
The speed of the passenger train is x+15=80mph

Let u be the amount needed to seed 5000 sf of lawn.
the ratio of seeds to square foot is constant.

hence:

60/3000 = u/5000

60/3 = u/5

20 = u/5

100 = u

To seed 5000 sf of lawn we will need u=100oz of grass seeds.