About the warning triangle when breaking down on the hard shoulder (Book p.169): which statements are correct? (multiple answers possible)
In that case the triangle no longer serves any purpose either, so you might as well leave it where it is.
If the car is already clearly visible with blinking hazard lights, placing it is not mandatory.
The distance is about 30 metres behind the car, and that applies on the motorway too — no separate, larger figure.
Explanation
False — trap: mandatory-equals-useful inversion. Not mandatory does not mean useless: the book still calls placing it 'wise' in that case (Book p.169).