Do #20 and if you want you can try #21

20: J: first I divided 2,400 by 4 to see if she had paid more or less than 25% , she paid less meaning her interest was 14%. Then I used the interest formula to see which answer was right: I=prt, for the principle use the difference between how much it was and what she paid, rate is .14, and time was the different time amounts.
21: she adds $100 a month and there are 12 months a year so $1,200 times .035. And she needs a total of 8,893 times 4, sorry that's all I know on that one :)