On a shelf, books with green cover and that with brown cover are in the ratio 2:3. If there are 18 books with green cover, then the number of books with brown cover is

As the simplest form of the ratio is 2:3

Therefore the common factor of 2 and 3 can be taken as x.

Therefore 2x = 18

x = 9

Therefore number of books with brown cover = 3x9 = 27