Do $\widehat{A}+\widehat{Y}=120^{\circ}$ và $\widehat{A}-\widehat{Y}=40^{\circ}$ nên $\widehat{A}=80^{\circ}$ và $\widehat{Y}=40^{\circ}$ (1)
Mặt khác, $\Delta ABC=\Delta XYZ$ suy ra : $\widehat{A}=\widehat{X}, \widehat{B}=\widehat{Y},\widehat{C}=\widehat{Z}$ (2)
Từ (1) (2), ta có: $\widehat{A}=\widehat{X}=80^{\circ},\widehat{B}=\widehat{Y}=40^{\circ};\widehat{C}=\widehat{Z}=60^{\circ}$