Knowing that a straight line is 180° you can use this to your advantage. If you take all parts of the ratio and add them together you can get the total number of "pieces" the 180° will be divided into.
Then divide the 180° by the total.
Now you can use the value for one "piece" and multiply it by how ever many the ratio says.
The angle FOG has the 4 value of the ratio so you multiply 20 by 4 to get the value of the angle.
m<FOG = 80°