In madurodam, one of the models of a windmill is 1.2 meters tall and 0.8 meters wide. the width of the actual windmill is 20 meters. what is its height?

Ratio ( model measure to actual measure)=model measure / actual measure
Data:
model width=0.8 m
actual width=20 m

Ratio=0.8 m / 20 m=0.04

ratio= model height / real height ⇒     real height=model height / ratio

Data:
ratio=0.04
model height=1.2 m

Therefore:
real height=1.2 m / 0.04=30 m

the height is 30 m. 

Other method:

model width/actual width=model height / actual height
Data:
model width=0.8 m
actual width=20 m
model height=1.2 m

Therefore:
0.8 m / 20 m=1.2 m / actual height
0.04 =1.2 m/ actual height
actual height=1.2 m / 0.04 =30 m

the height is  30 m