If $$5$$ apples cost as much as $$16$$ pineapples, then $$15$$ apples cost as much as pineapples. 
Overseas Competition->Knowledge Point->Calculation Modules->Basic Concepts of Equation->Equivalent Substitution->Algebraic Expressions
$$5$$ apples = $$16$$ pineapples  $$5\times3$$ apples = $15$ apples  $$16\times3$$ pineapples = $48$ pineapples 