How much does it cost to send a letter from san francisco to uk?

How much is it to mail a standard size and shape letter (eg of 10-20 grams in the UK, or under an ounce in the US), using airmail if the distance makes this appropriate, from San Francisco, officially the City and County of San Francisco, the fourth most populous city in California and the 12th most populous city in the United States, with a 2009 estimated population of 815,358 to the United Kingdom of Great Britain and Northern Ireland?

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Show reasoning Reasoning

I followed this chain of reasoning:

By calculation (generator: ["now@trueknowledge.com"]) I know that:
The current time is May 23rd 2013, 13:22:46

I know from locally stored knowledge that:
San Francisco, California is in the United States ([fact: ["137384348@trueknowledge.com"]])

the United Kingdom is in US postal Group 5 ([fact: ["162571234@trueknowledge.com"]])

Fact 1: 98 US cents is the cost of sending a letter between the United States and US postal Group 5 ([fact: ["653194780@trueknowledge.com"]])

Fact 1 is true for December 13th 2009, 00:00:00 onwards ([fact: ["653194784@trueknowledge.com"]])

By calculation (generator: ["cost_sending_letter_between_places@trueknowledge.com"]) I know that:
Fact 2: 98 US cents is the cost of sending a letter between San Francisco, California and the United Kingdom

Fact 2 is true for December 13th 2009, 00:00:00 onwards

By calculation (generator: ["timeperiodtopoint@trueknowledge.com"]) I know that:
Fact 2 is true at May 23rd 2013, 13:22:46

I used the following facts to provide this answer:

San Francisco, California is or has been in the United States

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the United Kingdom is or has been in US postal Group 5

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98 US cents has been the cost of sending a letter between the United States and US postal Group 5 since at least December 13th 2009, 00:00:00

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