The table below presents an aligned extract of verses in Gn44-20a, Gn42-12 Both verses discuss Joseph's status. The alignment justifies the Rashi assertion that It was unknown what happened to Joseph. But Judah lied and said he was dead because he was afraid if he said he was not to be found that they would demand Joseph be found.

Verse	Text of Verse	Rashi comment
Gn44-20	And we said to my lord, We have a father, an old man, and a child of his old age, a little one; and his brother is dead, and he alone is left of his mother, and his father loves him.	Here Judah says Joseph is dead.
Gn42-13	And they said, Your servants are twelve brothers, the sons of one man in the land of Canaan; and, behold, the youngest is this day with our father, and one is not to be found.	But the truth is that Joseph couldn't be found. Judah lied and said he was dead because he feared that if he admitted the truth they would force them to find Joseph.

Advanced Rashi: We could perhaps also classify this Rashi as using the contradiction rule. Both approaches yield the same result.