First, we know that all supplementary angles add to 180°. Then, since the angles are in the ratio 4:5, we can also represent these angles as 4x and 5x respectively. Both angle measures have the same variable (x) because no matter how large the angles are, they must have a common multiple because they are in a ratio. We can then write the formula 4x+5x=180 and solve for x. First, you combine like terms to get 9x=180. The you divide both sides by 9 to get x=20. Then, we can find both angles to be 80 (4*20) and 100 (5*20) by plugging them into 4x and 5x.
Got the answer from sdmatera (i’m giving creds)