Question

a bus decrease its speed from 80kmh to 60kmh in 5s

Answer 1

If you wanna find the retardation, it is -1.12m/s².
First convert it to m/s then subtract v1 from v2 and divide it by the time, it's 5s here.

For conversion to m/s, multiply the speed in km/h by 5/18.

Answer 2

Initial velocity = 80km/h =

80×1000/60×60= 22.22m/s

Final velocity = 60km/h =

60×1000/60×60= 16.67m/s

Acceleration = v-u/t

= 16.67-22.22/5

= -1.11 m/s^2