distance = speed * time

speed = distance / time

let 't' represent the time flown by the jet, then 't+2' represents the time flown by the propeller plane

let 'v' represent the velocity of the propeller plane, then 'v+85' represents the velocity of the jet

3400 = t(v+85)

3400 = (t+2)v, so v = 3400(t+2)

substituting 'v' with '3400/(t+2)' in the first equation:

3400 = t((3400/(t+2)) + 85)

solving for 't' we have a positive value of t=8

8+2 = 10

now we solve for speed:

for the jet, speed = 3400/8 = 425

for the propeller plane, speed = 3400/10 = 340

check: 425 - 340 = 85

the jet flies 8 hours at a speed of 425 mph

the propeller plane flies 10 hours at a speed of 340 mph