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"What's the number before infinity?" asks Claudia, aged four. We challenge Johnny Ball, legendary British TV presenter, to explain.

And in celebration of the voice of Sesame Street's Count von Count, Jerry Nelson - who has died aged 78 - there's another chance to hear our 2009 interview with the Count, in which he revealed his favourite number - 34,969.

Presenter and producer: Ruth Alexander.

(Image: Johnny Ball explaining infinity to a four-year-old Claudia)