If $$\blacktriangle +\blacktriangle =\blacksquare$$ and $$\blacksquare +\blacktriangle=●$$ and $$◆=●+\blacksquare +\blacktriangle$$, how many $$\blacktriangle$$s are equal to $$◆$$? 
Overseas Competition->Knowledge Point->Calculation Modules->Basic Concepts of Equation->Equivalent Substitution->Object Substitution