New Scientist Puzzle 73 – Changing Guard
by Rob Eastaway
From Issue #3296, 22nd August 2020 (link)
Fifteen members of the King’s 99th Dragoons are standing on parade. “Right turn!” screams the sergeant, and each soldier makes a 90° turn. Unfortunately, many of the squad struggle to know their left from their right. After this manoeuvre, five soldiers end up facing left, including Private Perkins, who is in the middle of the row.
X X X X X X X P X X X X X X X
Any soldier that ends up face to face with another soldier now does a 180° turn. This awkward ritual continues until no soldier can see another soldier’s face.
Can we be sure that Perkins will end up facing the right way?