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.