Answer:
Ending Balance Due = $14,980.106 
Total Interest = $780.106
Explanation:
solution
Total Interest and Ending balance due by the U.S. Rule are as given below
so interest is here for 70 day with 12 % of 21000
interest = 0.12 × 21000 ×  
interest = $490 
so 
payment = $2400 - $490 
payment = $1,910 
and adjusted balance  will be after that 
adjusted balance  = $21,000 - $1,910 
adjusted balance  = $19,090 
and
on 100 day
Interest  =  0.12 × $19,090  ×   
 
Interest  =  $190.9 
and
Payment  = $4,400 - $190.9 
payment = $4209.1 
So 
adjusted balance  = $19090  - $4209.1 
adjusted balance  = $14,880.9
and interest = $14,880.9 × 0.12  ×   
 
interest = $99.206
so Ending Balance Due  will be 
Ending Balance Due = $14,880.9 + $99.206 
Ending Balance Due = $14,980.106 
and
Total Interest = $490 + $190.9 + $99.206 
Total Interest = $780.106