transformer fcan and fcbn taps question

[omega83]

heres another question:

heres just the info I remember ..... your primary voltage is tap at 480v . you have 4 taps at 2-1/2% FCBN and 2taps at 2-1/2% FCAN. The building 480v voltage is not true and is at 460. how do you change the taps so you will have good voltage for 208v on secondary?

1)2-1/2 FCBN
2)2-1/2 FCAN
3)5% FCAN
4)5% FCBN
5)7% FCBN

I guess on this wit answer#3. how do you guys figure this out?

 

[jim dungar]

Think of:

AN = use these taps, if you have above normal incoming voltage
BN = use these, if you have below normal incoming voltage

 

[omega83]

Think of:

AN = use these taps, if you have above normal incoming voltage
BN = use these, if you have below normal incoming voltage

I kinda did research on it and yeah your right. For the answer

2.5 %fcbn 0f 480 is 468v
5%fcbn of 480 is 456V

ratio of 460 to 208 v is 2.211/1

so what I did was 468/2.211= 211v
456/2.211=206v


I think the answer should be 5%fcbn. Did I do this correct