Sahil went on a trip with his friends. They rented a car and travelled first 90 km at a speed of 60 km/hr, next 150 km at a speed of 80 km/hr, and final 210 km at a speed of 100 km/hr. What was the average speed for the whole distance travelled by them?

Using the average speed formula of total distance divided by total time, the time taken for the first part is 90/60 = 1.5 hours, the second part is 150/80 = 1.875 hours, and the third part is 210/100 = 2.1 hours. The total distance is 90 + 150 + 210 = 450 km, and the total time is 1.5 + 1.875 + 2.1 = 5.475 hours. Dividing 450 by 5.475 gives an average speed of approximately 82.19 km/hr.

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