When eight times an unknown number is decreased by 208 the result is four times the unknown number.

I think

This is your typical algebra question where you are looking for an unknown value given some information. The information can be set up as:
8 - x = 4*(x +3)

Distribute the 4:
8 - x = 4x + 12

Rearrange to get variables on one side:
8 - 12 = 4x + x

Simplifiy:
-4 = 5x

Isolate the variable:
-4/5 = x

Agc and bgd are vertical angles