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MY WORD processor has developed a fault. There’s one particular digit which it will not type: if you press the appropriate key absolutely nothing happens.

I can, for example, type the cube of 4 correctly, but not the square. Whereas for the larger number 7 8 (whose number of digits is less than the one-figure number I cannot type) I can correctly type both its cube and square.

Which digit do I keep missing out?

A £10 book will be awarded to the sender of the first correct answer opened on Thursday 3 August. The Editor’s decision is final. Please send entries to Enigma 829, New Å®ÉúСÊÓƵ, King’s Reach Tower, Stamford Street, London SE99 0BB. The winner of Enigma 823 was Mary Winning of Downham Market, Norfolk.

Answer to Enigma 823

Double sum

645368