At the end of 2005, the national debt of the u. s. was about 10^12 dollars, and the population of the u. s. was about 10^8. about how much was the per capita ( per person ) debt?

It is one of the scariest things I know.

According to your numbers, you use debt/American population.

10^12 / 10^8 = 10,000 
Put it into your calculator as 
1 *10 y^x or ^ or x^y 12 ÷ 1 * 10^8 = and it should bring back 10000.

That number is quite low. The American Federal Debt is now 21 000 000 000 000  and there are 330 000 000 Americans currently living on the planet. That means the debt is 63,600 for every man woman and child now in the US: each one of them owes this much. Most countries in the western world are no better off. Russia is the only country I know of that does not have a currency problem.