$$5$$ apples = $$16$$ pineapples  $$5\times3$$ apples = $15$ apples  $$16\times3$$ pineapples = $48$ pineapples 
If $$5$$ apples cost as much as $$16$$ pineapples, then $$15$$ apples cost as much as pineapples. 
1. $$6$$ 
2. $$18$$ 
3. $$21$$ 
4. $$48$$ 