1.2km/hr x 1/2 hr = 0.6km
At the starting point of X, A abd B jog in opposite direction.
After 1/2 hr, B jog further by 0.6km. Total distance apart is 5.2km
(5.2 km-0.6km)/2 = 2.3km
In 1/2 hr, A jog 2.3km, so his average speed will be 4.6km/hr.