Shawn Bixby borrowed $21,000 on a 120-day, 12% note. After 70 days, Shawn paid $2,400 on the note. On day 100, Shawn paid an additional $4,400. Use ordinary interest. a. Determine the total interest use the U.S. Rule. (Do not round intermediate calculations. Round your answer to the nearest cent.) Total interest $ b. Determine the ending balance due use the U.S. Rule. (Do not round intermediate calculations. Round your answer to the nearest cent.) Ending balance due $