Answer
a). $1.08s$
b). $1.04s$
Work Step by Step
$v=331+0.6T$
At $20^{\circ}C$, $v=331+(0.6)(20)=343m/s$
Speed of sound in steel = 4500m/s
a). d= 400m
Rail: $t=\frac{400}{4500}=0.09s$
Air: $t=\frac{400}{343}=1.17s$
Time lapse = $1.17-0.09=1.08s$ will elapse when the second will reach the person.
b). Speed of air = 36km/hr = 10m/s
Actual speed of sound = 343+10=353m/s
Air: $t=\frac{400}{353}=1.13s$
Thus, time lapse = $1.13-0.09=1.04s$
