It takes 15 months to pay off $1,500, @ $100 per month.
Interest of 1% () on OUTSTANDING BALANCE is also paid monthly. 
For total interest paid over 15 months, we get the sum of an AP, which begins with a 1st interest payment of $140 $14 ($1,500 - 100) * .01 (1%), 
and decreases by $10 $1 each month thereafter, for 15 months, with last being 0, since the remaining $100 will be paid then. 
We then get: 
Sum of 15 monthly interest payments, or 

Thanks, Tutor @IKLEYN. 
I hastily made the calculations and didn't realize that I calculated 10% interest i/o 1%. I've made the corrections. Thanks again!

     Month	      Debt         Interest (in dolars)
       (end of the month)       paid at the end of the month
----------------------------------------------------------------
      1		      1400		14
      2		      1300		13
      3		      1200		12
      4		      1100		11
      5		      1000		10
      6		       900		 9
      7		       800		 8
      8		       700		 7
      9		       600		 6
     10		       500		 5
     11		       400		 4
     12		       300		 3
     13		       200		 2
     14		       100		 1
     15		         0		 0
---------------------------------------------------------------
                                       sum = 14 + 13 + 12 + . . . + 1 = 105.


ANSWER.  The sum of all interest payments is $105.