For boundary value analysis, the test cases should include the boundary values just inside and just outside the equivalence partition for the 13% interest rate range:
R100,000 (just inside the previous range)
R100,001 (start of the 13% range)
R500,000 (end of the 13% range)
R500,001 (just outside the range)
These values ensure that both the edges of the partition are tested.