Before lunch Peter and Heidi have walked for $60$ minutes covering
a distance which the sign indicates should take $1$ hour and $40$
minutes. So the pair take only $\frac{60}{100}=\frac{3}{5}$ of the
time indicated by the sign. They stop for $15$ minutes, then cover
the remaining distance in $\frac{3}{5}\times 75=45$ minutes. The
journey takes $60+15+45=120$ minutes in total, so they reach
Salzbau at $2$pm.

*This problem is taken from the UKMT Mathematical Challenges.*