Did this mean that sometimes extensive revisions are needed, and that at other times 25% new or revised problems are sufficient to get over the bar, or did you mean that a new edition must have both? On first reading I took it as the former.
I'm glad you don't condone making new editions just for changing a few commas
, yet. I must say, though, that altering 25% of the problems is no better, in some cases at least. I don't know what specific topics your books cover or at what level, but consider the following, which could easily be in a Freshman Chem book:
An important reaction in rocket propellants is the following:
NH₄ClO₄ + Al → N₂ + HCl + H₂O + Al₂O₃
a) Balance the above equation.
b) If it is desired to make 1 kg of this mixture, compute the masses of the reactants.
Now, one might change this so the student is required to find the reactant masses for
2 kg of mixture, and call it a revised problem. Or pick some other, similarly simple reaction for the same exercise. If one can do that to 25% of the end of chapter problems and call it a new edition of the book, that's really no better than diddling the commas.
