Responsa for Zevachim 80:5

Q. Dough of a Gentile [which is exempt from the law of Hallah] was mixed with a piece of dough belonging to a Jew which by itself, was sufficient to become subject to the law of Hallah. How can the Jew take Hallah for, or from, the mixture?
A. In order to make sure that he takes Hallah from dough that is subject to that law, the Jew must take as Hallah a piece that is larger than the Gentile's dough.
SOURCES: Cr. 40; Pr. 423; P. 225; cf. Am I, 59.