Question

A car reduces its speed from 72 kmph to 36 kmph within 20 seconds.calculate its retardation.

Answer 1

$72kmph = 72 \times \frac{5}{18} \\ u = 20metre \: per \: second \\ 36kmph = 36 \times \frac{5}{18} \\ v = 10metre \: per \: second \\ t = 20seconds \\ a = \frac{v - u}{t} \\ = \frac{10 - 20}{20} \\ = \frac{ - 10}{20} \\ = - 0.5$
So, retardation is 0.5m/s^2.

Answer 2

76-36
40 kmph
them time is 20 sec
40/20 is equal to 2
so 2kmph is the retardation