Explanation:

Let the speed of the car starting from A and B be 'a' and 'b' respectively.

Take the first case. Both cars travel in opposite directions.

Relative speed = a + b

Total distance travelled = 120 km

Time taken = 1 hour

Distance = speed × time
⇒ 120 = a + b ---------> (1)

Take the second case. Both cars travel in same direction. Since A meet B, A is faster than B :-

Relative speed of the faster car = a − b

Total distance travelled = 120 km

Time taken = 6 hours

⇒120 = 6 (a − b)

⇒ 20 = a − b ----------> (2)

(1) + (2) gives :-

2a = 140

⇒ a = 70

Speed of car starting from A = 70 km/hr