Depends on your country.

Visit your countries government website. There should be a list of minimum wages for apprenticeships etc.

30% off of minimum wages

= 30% discount applied to the minimum wages

= minimum wages - (30% * minimum wages)

See related link for the full list of 197 countries and territories and their minimum wages:

Not all countries have minimum wage legislation. You would need to check the country concerned.

No, it is not mandatory. It may or may not be included in minimum wages