Well if we know he can walk 5km in one hour (or 60 minutes) we can do ratios,,,
Km : minutes
5 : 60
: 15
We can do 5 divided by 60 multiplied by 15 to get the unknown km
This will give us 5/4 or 1.25
So he can walk 1.25 km in 15 minutes.
Hope this helps! Any questions let me know :)