“…There are several other problem characteristics that can influence the difficulty of proportional problems. First, proportional problems in which a one-to-many correspondence (i.e., a unit ratio) is given are easier to solve than proportional problems in which only a many-to-many correspondence is given (Riehl & Steinthorsdottir, 2019; Vanluydt et al, 2018, 2020). In the case of one-to-one correspondence, an item in one set is related to an item in another set (e.g., in a group of children, each child gets one apple), whereas in one-to-many correspondence, each item of one of the sets is related to a specific number of items (more than one) in the other set (e.g., in a group of children, each child gets three apples), and in the case of many-to-many correspondence, a specific number of items (more than one) in one of the sets can be related to a specific number of items (more than one) in the other set (e.g., a group of three children gets altogether six apples; Bakker et al, 2014; Nantais & Herscovics, 1990).…”