Basic College Mathematics (9th Edition)

The unknown angle is 145$^{\circ}$.
The sum of the angle in a Triangle is 180∘. Here we will solve for the unknown angle. We will label it X. Therefore: 180$^{\circ}$ = 20$^{\circ}$ + 15$^{\circ}$ + X X = 180$^{\circ}$ - (20$^{\circ}$ + 15$^{\circ}$) X = 180$^{\circ}$ - (35$^{\circ}$) X = 145$^{\circ}$