If we call x the number of current shelves and y the number of smaller shelves, then 50x will be the current number of books distributed on the x shelves and 30y would be the number of books on y shelves. Since the # of books is the same in both cases, we have 1 linear equation: 50x = 30y

The question asks for how many of the smaller shelves are needed: What is y?

Statement 1:
y (# of smaller shelves) is 6 more than x (# of current shelves), so y = x+6. Note that this gives us a 2nd linear equation. Combined with 50x=30y, we have 2 distinct linear equations and 2 variables, so we could solve for y (and x) and thus answer the question.

[spoiler](1) is sufficient[/spoiler].

Statement 2:
x = 9. Plugging that into 50x = 30y would allow us to find y.
[spoiler]
(2) is sufficient[/spoiler].

D is correct.

In the shelve with 50 books each
Total books = 50X

In the shelve with 30 books each
Total books = 50Y

Both of these should be same i.e 50X = 30Y.

i) y = x+6
50X = 30(X+6)
X = 9
Sufficient

ii) X = 9
50X = 30 Y
Replace x value
Y = 15.

Both A& D are sufficient