Bheja fry question

Today I have come with the brain teaser.

B                   G                 N

D                   J                   R

G                   N                  ?

Usually these type of brain teasers follow one perticular mathematical series between them.

We just have to figure it out. 

Now in the first row, first letter is B and next letter is G. There is a difference of 5 letters. Next letter is N. There is a difference of 7 letters.

Series goes like this   5 , 7 

Now in the second row,  first letter is D. Next letter is  J. Difference is 6 letters between the two.  Next letter is R.  Difference between the two is 8 letters.

Series is  6, 8 

In the third row, it is   G  and N. Difference between the letters 7. We don't know the third letter. 

We got 3 series 

5  ,  7  

6  , 8

7 , ? 

In the third row .  Difference between the first two letters is 7 letters.

As in the above 2 equations, difference between the letters increases by 2 more letters than the previous one. 

The first difference was of 7 letters. 

So the unknown letter should be 9 letters from the last letter  N which is W.

The solved puzzle is

B                 G                     N 

D                  J                     R 

G                  N                    W