To express the federal debt of Country C, which is $278,560,125,000, in the form of a single digit times an integer power of 10, we look for a way to write this number in scientific notation.
The number is approximately 2.78560125 × 10^11. When rounding to one significant figure, it simplifies to approximately 3 × 10^11.
Therefore, the closest estimation of this amount is:
3 × 10^11.