If $$8$$ watermelons cost as much as $$12$$ pears, then $$24$$ waternelons cost as much as pears. 
Overseas Competition->Knowledge Point->Heuristics Skills-> Equivalent Substitution
$$8$$ watermelons = $$12$$ pears  $$8\times3$$ watermelons = $24$ watermelons  $$12\times3$$ pears = $36$ pears 