AVG SPEED = Total Distance / Total Time

AVG Speed = 240 km/hr

Total Distance = 4 Km

Total Time = 2/120 + 2/X (assume X is the speed for next 2 KM)

So we have

240 = 4/(2/120 + 2/X)

or

60 = 1/(2/120 + 2/X)

or

1+120/X = 1

which is impossible...

So answer is **Impossible**