The speed of a car was `50 km//hr` for the first 900s, then 40 km/hr for the 50 km and then the car decelerated uniformly at `10 km//hr^(2)` till it came to rest. The average speed of the car was :

Options

A. 50 km/hr
B. 7.2 m/s
C. 30 km/hr
D. 9.0 m/s

Answer

B

Explanation

Av. Speed `= ("Total distance")/("Total time")`
`= ((v_(1)t_(1) + v_(2) t_(2) + (v_(2)^(2))/(2a_(2)))/(t_(1) + t_(2) + (v_(2))/(a_(2))))`