Question

Two parallel lines are crossed by a transversal.

Parallel lines c and b are cut by transversal a. On line c where it intersects with line a, the uppercase left angle is 80 degrees. On line b where it intersects with line a, the bottom right angle is y degrees.
What is the value of y?

y = 40
y = 80
y = 100
y = 120

Answer 1

y = 80

Reason: The upper-left angle at line c corresponds to the upper-left angle at line b (corresponding angles with parallel lines), so that angle is 80°. At the intersection on line b the bottom-right angle is vertical to the upper-left angle there, so it is also 80°.