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Use the time line to calculate the following:
Anthony borrowed R3000 on the 04 February 2020 at 15% simple interest rate per annum.
He paid back R1000 on the 21 April 2020, R600 on the 12 May 2020 and R700 on the 11 June 2020.
How much did he still owe on the 15 August 2020, if all payments received the same interest rate
as charge on the original debt of R3000.
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I will give you two ideas, that are the keys for solutions this and similar problems.
(1) As you should know, in this problem with simple interest, the debt grows linearly with time with the slope of 15% = 0.15
debt = 3000*(1 + 0.15*t), where t is the time, in years.
As Anthony pays it by parts in different days, Anthony should not make distinctions,
which part of his payment goes for the principal and which part goes to cover percentage.
It is automatically accounted in the total current debt value.
(2) To count the days from date to date, use free of charge special online solvers from the Internet.
In the Internet, there are several such solvers for common use, for example, these ones
https://planetcalc.com/273/
https://ecampusontario.pressbooks.pub/businessmath/chapter/calculating-the-number-of-days-between-two-dates-in-ms-excel/#:~:text=To%20find%20the%20number%20of,between%20the%20two%20dates%20entered.
(last one is the Excel function)
Having said it, let' start solving.
(1) From 04 February 2020 to 21 April 2020, there are 77 days.
(notice that I do not going into details, if the endpoint days are included or not;
how many days are working days and how many holydays are there - it is irrelevant info now).
So, the debt at this day is 
= 3094.93 rands.
After paying R1000, the current debt on April 21, 2020 is R3094.93 - R1000 = R2094.93.
After April 21, 2020, the debt grows with the annual rate of 15% from R2094.93.
(2) From April,21 2020 to May 12, 2020, there are 21 days.
(again, I do not going into details, if the endpoint days are included or not;
how many days are working days and how many holydays are there - it is irrelevant info now).
So, the debt at May 12, 2020 is 
= 2113.01 rands.
After paying R600, the current debt on May 12, 2020 is R2113.01 - R600 = R1513.01.
After May 12, 2020 the debt grows with the annual rate of 15% from R1513.01.
(3) From May 12, 2020 to June 11, 2020, there are 30 days.
So, the debt at June 11, 2020 is 
= 1531.66 rands.
After paying R700, the current debt on June 11, 2020 is R1513.01 - R700 = R813.01.
After June 11, 2020 the debt grows with the annual rate of 15% from R813.01.
(4) From June 11, 2020 to August 15, 2020, there are 65 days.
So, the debt at August 15, 2020 is 
= 834.73 rands.
ANSWER. The debt at August 15, 2020 is 834.73 rands.
Solved.
