Notice that AD is a diameter, therefore,
[tex]x+y+55=180[/tex]
Furthermore, arcAB=25° and x° is the central angle that defined arcAB; thus, x°=25°.
Finally,
[tex]\begin{gathered} \Rightarrow25+y+55=180 \\ \Rightarrow y=180-55-25=100 \\ \Rightarrow y=100 \end{gathered}[/tex]
Thus, the answer is y=100
