一道英语的智力测试,

一道英语的智力测试,
Grandpa:You give me three empty bottles,and I'll give you a bottle of orange.
Dad:Give you twenty-one bottles of orange,Tom.
Tom:I drink all these bottles of orange,and give the bottles to Grandpa.Then he gives me some bottles of orange.How many bottles of orange do I drink in all(一共）?