Answer

I trust you have read the definition of interest as stated in 1.0. For the purpose of demonstration I am assuming the current Reserve Bank rate (commonly known as the "repo rate") stands at 10.0%. This is published on the financial page of most newspapers and is instantly available on the Web - just search for "repo rate". Note that the rate can change each month so check before calculating the default interest due.

Assuming the "default" amount due = R100 000, the late payment period is 21 calendar days the calculation is as follows:
In terms of 31.11 the default interest rate will be 10% x 160% = 16.0%.

Therefore the total amount due is:
R100 000 + (R100 000 x 16.0% x 21 / 365) = R100 920.55

Straightforward? I trust so!