## Geometry: Common Core (15th Edition)

$x = 3$
We know that the three interior angles of a triangle equal $180^{\circ}$. Let us set up the equation to add the three angles together. One angle measures $20x + 10$, the second measures $x - 20$, and the third measures $x + 25$: $(20x + 10) + (30x - 2) + (7x + 1) = 180$ $20x + 10 + 30x - 2 + 7x + 1 = 180$ Group like terms on the left side of the equation: $(20x + 30x + 7x) + (10 - 2 + 1) = 180$ Combine like terms: $57x + 9 = 180$ Subtract $9$ from each side of the equation to isolate the $x$ term: $57x + 9 - 9 = 180 - 9$ Subtract to simplify: $57x = 171$ Divide both sides by $57$ to solve for $x$: $x = 3$