Problem 5.40

There are two thin symmetrical lenses: one is converging, with refractive index $n_{1}=1.70,$ and the other is diverging with refractive index $n_{2}=1.51 .$ Both lenses have the same curvature radius of their surfaces equal to $R=10 \mathrm{~cm}$. The lenses were put close together and submerged into water. What is the focal length of this system in water?