In this study we aimed to inquire into the impact of the use of early arithmetic strategies by a group of fifth-grade students, on their solving of equations involving two representations of unknowns. Pre- and post-tests consisting of equation-solving items involving two representations of unknowns (number sentences containing empty 'brackets', as in the example 5 + (·) = 10, or equations containing x, as in 5 + x = 10), were administered to 126 fifth-grade students in a regular class setting. We found a notable difference between students' success rates on these two types of equations and their strategy use. Most students used the inversing strategy (arithmetic operations) after formal instruction on equation solving. Several students even used both the inversing and formal strategies (performing the same operation on both sides) for the same equation. When the unknown x appeared as the subtrahend or the divisor, the success rate dropped dramatically, and students tried to use the formal solving method of performing the same operation on both sides to solve such equations. The findings of this study not only suggest how teachers can be sensitive to students' different interpretations of unknowns, but also highlight the importance of using students' prior sense making to teach equation solving and of helping students gain an in-depth understanding of equation solving and representations of unknowns.