The purpose of this study was to find the level of abstraction that facilitates transfer in mathematical problem solving. Two experiments in this study showed that subjects who made good abstraction showed better transfer (Experiment 1), and it is possible to teach an abstracted schema quickly (Experiment 2), although a hint is necessary in testing. The abstracted schema was the idea of how to construct correct equations for target problems. This schema was at an more abstract level than the form of equations. Thus, we argue that the process named solution compression, in which two or more equations are considered to be constructed from one idea, is needed in order to generalize this schema and to promote transfer in mathematical problem solving.