The major objective of this study was to determine whether quantitative and qualitative reasoning are related, and, if so, which kind of instruction promotes their relation. The study had a pre-test/post-test design with two parallel groups. Both groups solved quantitative and qualitative problems, but while the QN group was only taught how to solve quantitative problems, the QL group was exclusively taught how to solve qualitative problems. The study used a sample of 68 ninth-grade Gymnasium (high-school) students of average mathematical abilities. The students solved multiple proportion problems. The study showed that: (a)~the examined problems were hard for most students; (b)~even in the QN group, quantitative reasoning was not improved; (c)~qualitative reasoning was improved in both treatment groups and the QL group scored better; and (d)~only the QL treatment related quantitative and qualitative reasoning.