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