Help needed

[Curlytop]

I do cash register systems including stock control for a living. The correct formula linking selling price, margin and cost price (disregarding such trivialities as VAT or other taxes) is:

S = C * 100 / (100 - M)

The important distinction is between margin and mark-up. Margin is profit as a fraction of the selling price, mark-up is profit as a fraction of the cost price. Most stock control systems are designed to work on margins but some of them (not ours) implement mark-up in error since that is simpler to handle.

The times I've had to explain this distinction to our customers! The formula bears some resemblance to the formulas of relativity, so you can't get a margin of 100% (since that would make the selling price infinite) unless you start with a supplier's freebie (C = 0) in which case the entire selling price is profit irrespective of what you charge for it - analogous to a massless particle (e.g. a photon) always travelling at the speed of light irrespective of its energy.