Unlimited Games. How should Red play 51?
This is close and in days of old I would have chosen 20/14 unthinkingly. Now we recognise that it is important to keep the rear checkers connected if possible so they can create an anchor.
The best play is 24/23, 6/1*. This keeps the rear checkers connected, takes a tempo from White and duplicates ones. 20/14 is not quite an error but it is edging that way.
This concept of keeping the rear checkers connected has been a theme in problems this year so bear it in mind.