比如，A residential mortgage of $250,000 has just been arranged for 25 years at an interest 10%. It calls for payments to be made every two weeks.

what is the amount of each payment.

A residential mortgage compounds semi-annually.

10%/2=5%

semi-annual===26 weeks

one payment every 2 weeks, so you have to calculate the  effective rate for a 2- week period.

Because it is a compound interest , you can not just divide 13.

(1.05)^(1/13) --1 = 0.376% which is the "true" rate for  a 2- week period.

-----------------------------------------------That is the calculation for compund interests.------------------------

Now, if you have a finacial calculator, 0.376% would be the rate you use for all calculations related to this example.