plane 1 travels 395 miles in 1 hour
plane 2 travels 422 miles in 1 hour
together, they travel (395+422) 817 miles in 1 hour
to have 2451 miles between then, they'd need to travel
[tex]\frac{2451 miles}{817 miles}=3[/tex]
so in 3 hours, they'll be 2451 miles apart