My logic is a based upon a mental geometric construction.
I am heading 280. If the NDB had a relative bearing of 180 it would be behind me .. i.e. 100. But it's 160 therefore 20 degrees further north i.e 080.
Variation is west by 5 degrees so as I am going from a magnetic to a true bearing thats 5 off = 075.
If it's 075 to the beacon it must be 075 + 180 away from it = 255.