Explanation:
2 m corresponds to 0.4 cm on a blueprint.
so,
2 m / 0.4 cm = 200 cm / 0.4 cm
our fun the point of view of the blueprint
0.4 cm / 200 cm
the scale on a map or blueprint is usually normed to one unit on the blueprint :
0.4/200 × f/f = 1/(200×f)
0.4 × f = 1
f = 1/0.4 = 2.5
so, our scale is
(0.4 × 2.5) / (200 × 2.5) = 1 / 500
that means 1 cm on the blueprint corresponds to 500 cm or 5 m in reality.