Well, visually picture 2 lines bisecting each other. The opposite angle of ∠1 is ∠3 and the opposite angle of ∠2 is ∠4.
Remember, when all 4 angles are added together, it equals 180 :)
Also, remember that the opposite angles are always the same.
So, because ∠3 is opposite of ∠1, and ∠1 = 40°, ∠3 also equals 40°
So, simply subtract 80° from 180° because you add the two angles together.
180° - 80° = 100°
Divide 100° by 2, because there are 2 angles left :)
100° / 2 = 50°
So, ∠2 and ∠4 = 50°
SO YOUR ANSWER IS :
∠1 = 40°, ∠2 = 50°, ∠3 = 40°, ∠4 = 50°
~Hope I helped!~