homework help- simple interest application

[Ivanthedumbfck]

Weiner has a credit card that charges 19.8% per annum. On May 5, he withdrew a $200 cash advance. On May 15, he purchased a cell phone for a rice of $189, paying by credit card. On May 31, he received the card's statement, noting that the minimum balance is $15 and payment is due by June 10. If he pays the full balance on June 8, how much does he pay?

What i did so far:

May 15: I=200(0.198%)(10/365)=1.084931
Partial Payment: 189-1.084931=187.915078
Balance Remaining: 200-187.915078=12.08493

May 31: I=200(0.198%)(26/365)=2.820821
Partial Payment: $15-2.820821=12.179179
Balance Remaining: 12.08493-12.179179= -0.094249

It's all messed up, the final answer should be $391.93.

 

[Ishuda]

Using a 365 day year, interest is at a rate of 0.198/365 = i = 0.000542465753424658 per day.

You owe interest on the May 5th purchase from May 5th to the time of the bill [or possibly until you pay it, depending on just what is charged but let's use May31st, the time of the bill]. Assuming simple interest we have interest for 26 days or
Interest owed for May 5th advance
= 200 * 0.000542465753424658 * 26 = $2.82

You owe interest on the May 15th purchase for 16 days or
Amount owed for May 15th advance
= 189 * 0.000542465753424658 * 16 = $1.64

So total owed is 'borrowed' (=$200 + $189 = $389) plus interest ($2.82 + $1.64 = $4.46) or $393.46.

If the book answer is correct, I made a bad assumption. Those assumptions which might be wrong include, (1) I did the arithmetic correctly, (2) it was simple interest, (3) interest should be charged until May 31st, (4) 365 days are used for number of days in a year (per annum), (5) you stated the question properly and accurately, and maybe some more.

 

[Ivanthedumbfck]

Thx Ishuda for helping me . The solution wasn't from the book, it was from the professor and he may have gave the wrong solution.