This is a cheeky answer.

Assume the usual rules of arithmetic and logic.

We are told that 9 - 1 = 10, that is, 8 = 10, which is a contradiction.

By the rules of logic, since we have derived a contradiction, we can now derive anything else (like 'magic', cf the storybook character Minerva), such as, that

29 - 1 = 30.

QED

This is a cheeky answer.

Assume the usual rules of arithmetic and logic.

We are told that $9 - 1 = 10$, that is, $8 = 10$, which is a contradiction.

By the rules of logic, since we have derived a contradiction, we can now derive anything else (like 'magic', cf the storybook character Minerva), such as, that

$29 - 1 = 30$.

QED