Map scale conversion????????

How would you express a fractional scale of 1/50 000 on a map as a verbal scale in kilometres and centimetres?

Is it
(a) 1cm = 2 km
or
(b) 1 cm = 0.5 km?

If one is right, why? Please show me your calculations if you can.

(b)
(1 km/50 000 km)(1000 m/1 km)(100 cm/1 m)=(100 000 cm/50 000 km)=(2 cm/1km)=(1 cm/0.5 km)

Just remember to keep track of units and these conversion problems aren't so bad :)