Initial distance (D1):75 Km
Time taken to cover this distance= 3 hrs.
Speed= Distance /time
Speed1= 75/3= 25Km/hr
Later the train travels
Distance (D2):66 Km
Time taken to cover this: 1hr
Speed= Distance/time
Speed2= 66/1= 66Km/hr
Average speed= (speed1+ speed 2)/2
Average speed= (25+66)/2
Average speed= 45.5 Km/hr