PNP Plc JUN 07 and DEC 08 Gorwa

I am really struggling in calulating the net cost and benefit of offering a discount to receivables. I don’t know how to approach these types of questions. Even after looking at the Kaplan exam kit answers for these questions, I do not understand them. Does any one here has a set way to go about these questions? If yes, I would be grateful to know. Thanks in advance.