Black to play. Maric-Gligoric, Belgrade 1962. White has offered to exchange the rooks on f5 and c3. But Gligoric found a much stronger shot, prompting instant resignation. What did he play? Answers should be emailed to chess@spectator.co.ukby Monday 28 February. There is a prize of £20 for the first correct answer out of a hat. Please include a postal address.

Last week’s solution 1…Rxa7+ 2 Rxa7 Rb5+ Capturing the rook leads to stalemate, while otherwise it can check along the b-file forever.

Last week’s winner Simon Whatson, Thirtover, Berkshire