Solution:     

weight of object in air =$ w_{1}$ = 18 N
                     Weight of object immersed in water $= w_{2} = 11.4$ N
                      Density of glass =$ \rho = 1000 kgm^{-3}$

    The density of the object = D =?
    Nature of the material =? 

                                D = $\frac{(w_{1})}{(w_{1}- w_{2})\times \rho}$  
                                D = $\frac{(18)}{(18- 11.4)} \times 1000$    
                              =$\frac{18}{(6.6)} \times 1000$ $ = 2.727 \times 10^{3}$ = $2727 kgm^{-3}$ 

The density of aluminium is $2700 kgm^{-3}$, and the above-calculated value of density is 2727 $kgm^{-3}$ nearest to the density of aluminium, so the material of the object is aluminium.