I am trying to figure out how to calculate  the trade-weighted index of foreign real GNP for the US in relation to Japan, Mexico and the Phillipines.

Any thoughts? Your help is greatly appreciated. Thanks!

Supply and demand. Thats the only thing economics brings to my mind.

I guess you are trying to find out the foreign income in order to assess the export demand from those countries. If this is so, you should look at the trade % of USA with those countries. Then you can multiply that with the GNP of those countries to come up with a number that could be a representative for the foreign income which then determines the total export demand for the united states.

You got to think about the trade -weight here because even if a country has higher income, its importance to USA could be less if the demand for USA manufactured goods is less in that country.