Answer:
a = 80°, b = 100°, c = 80°, d = 100°
Explanation:
You know a + b + c + d = 360°.
Plugging b + c + d = 280° into the formula above:
a + 280° = 360°.
a = 80°.
You know a + d = 180° because of angles on a straight line. Solve for d:
80° + d = 180°.
d = 100°.
For the same reason, you know a + b = 180°. b must be equal to d, which is 100°.
Lastly, c + d = 180° because they make up the second line. Solve for c:
c + 100° = 180°.
c = 80°.
a = 80°, b = 100°, c = 80°, d = 100°.