Every worksheet here is produced when you ask for it. There is no library of files behind the site and no PDF that everyone downloads: the settings on the left describe the exercise, and the page you see is built from them, in your browser, as you change them. Pressing New set draws different numbers with the same settings, which is what makes the same page usable twice in a term.
Nothing is random at the moment of printing. Each sheet is built from a short seed that sits in the address bar next to the settings, and the same seed and settings always produce the same problems in the same order. That is what makes a worksheet bookmarkable, and it is why two versions of a test can be handed out on one row of desks and marked from one printout — each version carries its own seed and its own key.
Random numbers alone make bad worksheets: the same problem twice on one page, ten multiplications by one, subtractions that go negative for a class that has not met negative numbers, divisions that do not come out whole. Every generator on this site is built against rules that forbid those, and every rule is covered by automated tests that run before the site is deployed. A generator whose tests fail does not reach the site — that is a rule of the project, not an aspiration.
The answer key is not stored anywhere. It is recomputed from the same seed as the questions, so it cannot drift out of step with them, and it is laid out identically so it can be placed over a finished sheet for marking.
The preview is the page. It is the same markup the printer receives, drawn at the proportions of the paper size you picked, so there is no surprise when the sheet comes out. US Letter is the default because most of the people using this site are in the United States and Canada; A4 is one click away and the choice is remembered. Margins are set at 12 mm, which keeps the page inside the unprintable edge of a domestic inkjet.
Sheets print black on white with no fills and no background colour. A teacher printing thirty copies is paying for every drop of ink, and a decorative border costs more of it than the exercise does.
It does not ask you to make an account, it does not put a watermark across the page, and it does not keep the good settings behind a subscription. It also does not use cartoon characters on early years sheets — those belong to somebody else, and a page you are free to copy for a whole class should not carry another company's property.
A contact address will be published here before the site leaves preview.