Two identical glass (μe=3/2) lenses of focal length f each keeps in contact. The space between the two lenses is filled with water (μw=4/3). The focal length of the combination is.

The water in between two convex lenses in contact acts like a concave lens.
Focal length of convex lens is given by
1/f = (μe-1)(2/R) = 1/R
Focal length of concave lens is given by
1/f' = -(μw-1)(2/R) = -1/(3f) => f' = -3f
Equivalent focal length is given by
P = 1/feq = 1/f - 1/(3f) + 1/f = 4/(3f) => feq = 3f/4