Fractional relationships are difficult for students to understand. Perhaps this is because there are few non-contrived "real-life" situations that can be used as effective models for instruction of these somewhat abstruse concepts. Yet, there is a motivating, easily understandable, culturally relevant microworld which quite naturally utilizes fractional relationships and operations: Music!
Harris, J. (1986). Logo music: A numerical denomination. Logo Exchange, 5(1), 17-21.