A man covers half of his journey at 6 kilometre per hour and the remaining half at 3 kilometre per hour his average speed is

Find total time taken. Let total distance is 2x, hence first half distance will be covered in x/6 hours, similarly last half distance will be covered in x/3 hours. Now add both times taken. Total time taken is x/2 hours. Average speed = total distance/ total time => 2x/x/2 => 4 km/h is answer