If $$2$$ melons can serve $$15$$ people, I need $$4\times2=8$$ melons for $$4\times15=60$$ people. 
If $$2$$ watermelons can serve $$15$$ people, I need  watermelons for $$60$$ people. 
1) $$4$$ 
2) $$6$$ 
3) $$8$$ 
4) $$12$$ 