A bus starts from rest and acceleration at a uniform rate `4 m//s^(2)` for certain time. It then moves with a constant speed for some time and finally retards at `4 m//s^(2)` to come to rest. Average speed of the bus during the total journey is 15 m/s and the total time is 20 sec. Time duration for which the car moves with constant speed is:

Options

A. 18 sec
B. 16 sec
C. 12 sec
D. 10 sec

Answer

D

Explanation

`A = 4 m//s^(2)`
`t_(1) = t_(3)`

Total distance `s = 15 xx 20`
`s = 300 m`
`300 = 2 xx (1)/(2) xx 4 xx t_(1)^(2) + (4t_(1)) t_(2)`....(i)
`15 = (300)/(2t_(1) + t_(2))`.....(ii)