Answer
17,600 dollars
Work Step by Step
We know that the average debt is 25,600 dollars. Since 5/16 is borrowed in the last year, the amount borrowed in the first three years is:
$$ = 1- \frac{5}{16} = \frac{11}{16} $$
Thus, we find the amount that was borrowed in the first three years:
$$ = 25,600\times \frac{11}{16} \\= \frac{281600}{16} \\ = 17,600 \ dollars$$
