HKIMO 2017 Grade 6 National Final (Heat) Set 04 Question 12

How many reduced fractions greater than 0 and less than 1 have denominator 80? The positive-proper restriction is specified in the source glossary.

Source glossary (English–Vietnamese): Simplified fraction: phân số tối giản lớn hơn 0, nhỏ hơn 1; Denominator: mẫu số.