Question 22213
14% of $3900 = 0.14 x 3900 = $546


However, one must assume that the 14% is an annual rate, but the problem specifies only 90 days.  
90 days is approximately 1/4 of a year.
1/4 of 546 = $136.50 interest due.


After 90 days 3900 + 136.50 = $4,036.50 must be paid.