Answer:
∠F = 80
Step-by-step explanation:
Let's find inner ∠D first, as you can see, it's a straight line but a line is in between, the outer angle is 120, and a straight line is 180, so 180 - 120 = 60. The inner ∠D is 60, now we can find ∠F, a triangle is 180:
40 + 60 + ∠F = 180
100 +∠D = 180
subtract 100 from both sides
∠F = 80
