Distance = speed * time

the distance out = the distance back

240 T = 320 (35 - T)

we don't know how long it takes to get out there yet, but we know that if it takes T minutes out there it takes 35-T to get back because 35 is the total time flying.

we go ahead and expand this out

240T = 320*35 - 320T

560T = 320*35

T = 320*35/560 (you can punch those numbers in)

from the trip out there

x = 240 T

and you just plug the T you got above in there to get

240 * 320*35/560 km

you get one free. next time you have to give it a shot first and show your work.