If the value of $$x+2y$$ is $$3$$, then what is the result of $$\frac{1}{2}x+y-1$$?． 
Overseas Competition->Knowledge Point->Calculation Modules->Basic Concepts of Equation->Equivalent Substitution->Algebraic Expressions
$$\frac{1}{2}x+y-1=\frac{1}{2}(x+2y)-1=\frac{3}{2}-1=\frac{1}{2}$$． 