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In his review of David A. Weintraub's book How Old is the Universe? ("13.7 billion years - final answer?" 10 February), Shobhit Mahajan says the author takes seriously "Stephen Hawking's wry comment that every equation in a popular science book halves its sales".

In 1986, when I appraised the draft typescript of A Brief History of Time, I quickly concluded that it could not become a popular science best-seller because of the many equations. Stephen did not agree, but when he pressed me for a reason I responded with "...every equation halves the market", and that did it.

This incident is mentioned in the author's preface to the UK edition (Bantam, 1988).

Simon Mitton, St Edmund's College, Cambridge.