Explanation:
speed = distance / duration
therefore,
distance / speed = duration
as the speed is given in km/h, the duration is measured in hours. and distance in km.
to arrive 30 minutes (= 0.5 hours) later means it took us 0.5 hours longer.
distance/4 = distance/5 + 0.5
distance = 4×distance/5 + 4×0.5
5×distance = 4×distance + 5×4×0.5 =
= 4×distance + 10
distance = 10 km