Cities M and N are 600km apart Bus A starts from city M towards N at 9AM and bus B starts from city ...........

Explanation:

Bus A
Travels 200km @ 40kmph
the next 200km @ 50kmph and
the final 200km @ 60kmph
So, Bus A will be at a distance of 200km from city M after 5 hours, and at a distance of 400km after 9 hours, and reach N after 12 hours and 20 mins
Bus B
Travels at an overall average speed of 50kmph, so will take 12 hours for the entire trip.
So, Bus B will travel 160 kms in the first 4 hours , 200 kms in the next 4 hours and 240 in the final 4 hours
So, both buses cross each other when they are in their middle legs.
After 5 hours, bus A will be at a position 200kms from city M.
At the same time, bus B will be at a distance 210kms from city N (4 * 40 + 50).
The distance between them will be 190kms (600 - 200 - 210).
Relative speed = Sum of the the two speeds = 50 + 50 = 100 kmph.
Time taken = 190/100 = 1.9 hours. = 1 hour and 54 minutes.
So, the two buses will meet after 6 hours and 54 minutes.
Bus B will have travelled 210 + 95 = 305 kms.
So, the two buses will meet at a point that is 305 kms from City N and 295 kms from city A.
Hence, the buses cross 295 kms from city A.