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who paid for it and how much?
what percentage?

Only a little more then 60% of it
To date, the International Space Station has cost as much as $160 billion, with the
United States providing the bulk of the money — nearly $100 billion (although it depends how you include the price of the space shuttle program). Russia, Europe, Canada, and Japan chipped in for the rest. It's arguably the most expensive single object ever built.

if it were a corporation and you had 60% shares...would you have a problem doing what you wanted to do?

If the US pulls OUT..then what happens?

The fact notwithstanding is that anyone that needs to get to it, needs the US to give them a ride.

defacto ownership?