Average round-trip speed: 2/(1/80+1100) = 800/9 (km/h).

Set the distance to 400 kilometers.

Time to go: 400/80=5 hours.

Return time: 400/ 100=4 hours.

Round trip time: 5+4=9 hours

Round trip distance; 400+400=800 km

Average round-trip speed: 800/9 (km/h)