This problem belongs to comparing quantities.
we have 1 cm :5km
we have different units in our ratio so we need to convert km as cm so we can have same units here.
we know that
1 km =1000m
5km=5000m
100cm=1m
5000m=500000cm
1cm:500000cm =1÷500000 units will be cancels as we have same unit on to and bottom
1÷500000=0.000002 this is the simplified form of given ratio