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To find the average speed for the entire trip, we need to divide the total distance by the total time. Remembering that $d=r\cdot t$, and looking at each of the four parts of the trip, these pieces can be determined.

First, a car traveling at 40 kph for 20 km will be traveling for $20/40=.5$ hours. Next, a car traveling at 50 kph for 25 km will be traveling for $25/50=.5$ hours. Next, a car traveling at 60 kph for 45 minutes (.75 hours) will travel a total of $60\times .75=45$ km during that time. Finally, a car traveling 48 kph for 15 minutes (.25 hours) will travel a total of $48\times .25=12$ km.

The total distance traveled was $20+25+45+12=102$ km. The total time was $.5+.5+.75+.25=2$ hours. Therefore, the average speed of the car was $102/2=\boxed{51}$ kph.