30 weniger
180 km/h entspricht 100 %
30 km/h entspricht x %
$${\frac{{\mathtt{30}}{\mathtt{\,\times\,}}{\mathtt{100}}}{{\mathtt{180}}}} = {\mathtt{16.666\: \!666\: \!666\: \!666\: \!666\: \!7}}$$
Gruß "radix"