>>Another good one, Hilmar!
>
>BTW, this is the shortcut: 40 km/h x 1/2 h = 20 km.

Oh, yeah. The variation I'm familiar with has to do with a fly zooming back and forth at a speed of 100mi/hr. between two trains, each coming toward the other at 60 mi/hr from 120 miles apart. How far does the fly travel? That's a rhetorical question in this case. ;)