On internal rate of return, I have been using the 7-column analysis for computation of the IRR for redeemable debts. But one thing i would appreciate to get clarifications is what guides on determining the discount rates to use? I have been using the 5% and 10% for most of the cases, but i am not so sure why should i use them as these are not mentioned in the question, apparently in some other text books various rates such as 5% and 7% or 10% and 12% have been employed.