6. Tony Blair lives at 10 Downing Street. 7. Tony Blair lives on 10 Downing Street. The place 'number 10 Downing Street' is a spot. So it should be the sixth sentence. For example, we say I met him in London but I met him at Heathrow Airport. London is a huge geographical area where as the airport is a particular place though it has a mass area. More @Wikipedia
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