To solve that question we must remember that the sum of all internal angles of a triangle is 180°, we can say that

That's a rule! it's always true.
The problem says that

Then let's use it in our equation!
![\begin{gathered} \operatorname{\angle}X+\operatorname{\angle}Y+\operatorname{\angle}Z=180 \\ \\ 55+\operatorname{\angle}Z=180 \end{gathered}]()
Now we can solve it for Z

Therefore the measure of Z is 125°