Rate of walking

suppose his rate of walking is x km/hr

so time taken to walk 3 km is

distance/speed=3/x

if he had walked 1 km /hr faster, then, his speed would have been

(x+1) km/hr

with that speed the time taken to cover 3 km distance would be

distance /speed

=3/(x+1)

the diff bet these two timings is 15 minutes

whne he walks faster , he takes less time

so we get

time taken to walk at the rate of (x+1)km/hr -time taken to walk at the rate of x km/hr=15 minutes=1/4 hr

so