76% at most. The situation is: students who subscribed to "Required Reading for Primary School Students" also subscribed to "Mathematical World".

At least 6 1%. The situation is as follows: 1-76% = 24% of the students have not subscribed to Required Reading for Primary School Students, 1-85% = 15% of the students have not subscribed to Mathematical World, assuming that this 15%+24% = 39%.