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21岁子女必须和在美国的工作父或母住在一起至少半年吗? and filed the tax return together吗?
-Yes, thus the parent's income can be counted as the family income. Then, the child files I-864, and the parent files I-864A to allow the child to use the parent's income to sponsor the parent green card application. It seems strange, right? But that is the law.

分住在两个州行吗?
-Probably no. Consult this issue with a lawyer, but don't hire the lawyer until you are sure whether you can use the parent's income for him/her green card application in case you waste your money to the lawyer.
在美国的工作父或母给21岁子女付学费.