If $$2$$ dogs weigh as much as $$3$$ cats, and $$2$$ cats weigh as much as $$15$$ mice, how many dogs weigh as much as $$45$$ mice? 
Overseas Competition->Knowledge Point->Calculation Modules->Basic Concepts of Equation->Equivalent Substitution->Algebraic Expressions
In weight, $$45$$ mice $$=3\times (15$$ mice$$)=3\times (2$$ cats$$)= 2\times (3$$ cats$$)=2\times (2$$ dogs$$)= 4$$ dogs. 