If $$2$$ watermelons can serve $$15$$ people, I need  watermelons for $$60$$ people. 
Overseas Competition->Knowledge Point->Word Problem Modules->Solving Problems Involving Proportions->Dividing Quantities Based on Ratio
If $$2$$ melons can serve $$15$$ people, I need $$4\times2=8$$ melons for $$4\times15=60$$ people. 