A driver can stop his car from the red signal at a distance of 20 m when he is driving at 36 kmph and 41.25 m when he is driving at 54 kmph. Find his reaction time.

Explanation

Let t is reaction time of driver during which he moves with uniform speed
Distance covered during reaction time `=S_(1)=ut`
As he applies brakes, distance covered before coming to rest is
`S_2=u^2/(2a)`
Total distance covered is `S=ut+u^(2)/(2a)`
`20=10t+(100)/(2a).....(1)`
`41.25=15t+(225)/(2a)......(2)`
Solving above equations 3.75=7.5t
t=0.5s