Question 1098670
payments are 200 dollars at the end of each week for 15 years.
interest rate is 10.4% per year compounded weekly.
assuming 52 weeks in a year, you would translate this problem as follows for the online calculator at 


present value = 0
future value = 0
number of periods = 15 * 52 = 780 weeks
payment amount = -200
interest rate % per period = 10.4 / 52 = .2
payment made at end of time period


you would then click on PV and the calculator will tell you that the present value of the loan is $78,953.63


the total payments you made would be 780 * $200 = $156,000


the total interest you paid would be $156,000 - $78,953.63 = $77,046.37


the calculator i used is at <a href= "https://arachnoid.com/finance/" target = "_blank">https://arachnoid.com/finance/</a>


here's a picture of the results from this calculator.


<img src = "http://theo.x10hosting.com/2017/102310.jpg" alt="$$$" >


you could also have calculated manually using the following formula.


PRESENT VALUE OF AN ANNUITY WITH END OF TIME PERIOD PAYMENTS 

p = (a*(1-1/(1+r)^n))/r 

p is the present value of the annuity.
a is the annuity.
r is the interest rate per time period.
n is the number of time periods. 


in your problem:


a = 200
r = 10.4 / 100 = .104 / 52 = .002
n = 15 * 52 = 780


the formula of p = (a*(1-1/(1+r)^n))/r becomes:


p = (200*(1-1/(1.002)^780))/.002


the result would be p = $78,953.62978 which is equal to $78,953.63 rounded to 2 decimal places.


that's the same result that the calculator gave you.


a summary of financial formulas that you might find useful can be found at this link.


<a href = "https://www.algebra.com/algebra/homework/Finance/THEO-2016-04-29.lesson#f5" target = "_blank">https://www.algebra.com/algebra/homework/Finance/THEO-2016-04-29.lesson#f5</a>