SAT Question a6dbad6b: Lines, Angles, and Triangles answer and explanation with Desmos solution

Choice B is correct. Let the measure of the third angle in the smaller triangle be $a^{\circ }$. Since lines $l$ and $m$ are parallel and cut by transversals, it follows that the corresponding angles formed are congruent. So $a^{\circ }=y^{\circ }=20^{\circ }$. The sum of the measures of the interior angles of a triangle is $180^{\circ }$, which for the interior angles in the smaller triangle yields $a+x+z=180$. Given that $z=60$ and $a=20$, it follows that $20+x+60=180$. Solving for $x$ gives $x=180-60-20$, or $x=100$.