I often used to watch Think of a Number when I came home from school back in the 1980's, though it was first shown in 1977.
This was presented by the excellent Johnny Ball, who is an expert on maths and numbers.
Think of a Number was about children in a studio asking about how different things worked, asking Johnny who then told them in a simple way. Later series didn't have an audience in the studio.
Johnny Ball hosted similar shows to this as well, including Think Again and Think Backwards.
As I write this review, Johnny has just published a book of the same title to this.