Two Plano-convex lenses each of focal length 10cm& refractive index 3/2 are placed as shown. Water  $(\mu =4/3)$ is filled in the space between the two lenses. The whole arrangement is in air. The optical power of the system in dioptre is:

Equivalent focal from of the system of three lenses in contact is given as 
$\frac{1}{f_{eq}}=\frac{1}{f_G}+\frac{1}{f_W}+\frac{1}{f_G}$---------- (1) 
Where focal length of glass lenses is given as $f_G=\frac{R}{\mu -1}=2R$
$R=5cm$

Focal length of water lens is given as $f_W=\frac{R}{2(\mu -1)}=\frac{3R}{2}=7.5cm$
Now from equation-(1), we have $\Rightarrow \frac{1}{f_{eq}}=2\left(\frac{1}{10}\right)+(-\frac{2}{15})=\frac{1}{15}$
Equivalent optical power of the given system is given as $P_e=\frac{1}{15}\times 100=6.67D$