A and B start from the same point and run around a circular stadium, whose circumference is 4200 m, at the rate of 500 m and 700 m per minute respectively, in the opposite directions. They will meet each other in :

Let us assume A cover P meter in the time after which he meets with B.
According to question,
Time taken by A to cover the P distance = Time taken by B to cover the 4200 - P distance.
Use the distance and time formula
P / 500 = (4200 - P ) / 700
Or 700P = 500 x 4200 - 500P
Or 700P + 500P = 500 x 4200
Or 1200P = 500 x 4200
Or 12P = 5 x 4200
Or P = 5 x 4200 / 12
Or P = 5 x 350
Or P = 1750 meters
Required time taken by A to cover 1750 meters = 1750 / 500 = 175/50
Required time taken by A to cover 1750 meters = 3.5 minutes.