This is a series of puzzle caches based upon the UCI World Tour. Each puzzle builds on the next and they will get increasingly more challenging as you progress through the series. The puzzles will lead to a trail of caches along official cycle routes and therefore are best done by bike. The caches are small and will not contain swaps but could cope with a small TB or geocoin.
The Paris-Roubaix is my favourite race of the year. Nicknamed the Hell of the North the race includes 27 secteurs of pavée. The most famous is secteur 18 La Trouée d'Arenberg (The Arenberg Trench). Somewhere near one end of the trench is a tree with three numbers (ABA) written on it. It's on the side of the Trench with the pavement.
Subtract (A-B-1)(A+B+2) from your Northing for Stage 11.
Subtract (A+B+1)(A-B+2) from your Westing for Stage 11.
You can validate your puzzle solution with certitude.