Hey Matt512.

Hint: If you have a ratio of 1/25000, then going from map scale to real scale means you multiply by the inverse ration which is 1/(1/25000) = 25000. So 25000x10cm = 250000cm = 2500m = 2.5km.

Its always the case that if you have a ratio to go from one unit to another (in this case some form of metres to map units) then going in the reverse (map units to some form of metres) is done by calculating the reciprocal of the ratio.