50 books in each of the larger shelf. 30 books in smaller shelf.
smaller shelf?
1) small shelves = 6+large shelves
50x = 30(x+6)
50x = 30x + 180
20x = 180
x = 9
smaller shelf = 9+6 = 15
Sufficient.
2) 9 large shelves.
50*9 = 450
450/30 = 15 small shelves.
Sufficient.
Thus, D is the answer.
how many shelves needed ?
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Source: Beat The GMAT — Data Sufficiency |

















