You are given 3 lenses having powers P₁ = 6D, P₂ = 3D and P₃ = 12D. Which two of these lenses will you select to construct a microscope?

Given, three lenses.
Power of first lens, P₁ = 6 D
Power of second lens, P₂ = 3D
Power of third lens, P₃ = 12 D 

Magnifying power of a microscope is given by, 

                   $\boxed{\mathrm{m} \propto \frac{1}{{\mathrm{f}}_0}.\frac{1}{{\mathrm{f}}_{\mathrm{e}}}\propto {\mathrm{P}}_0{\mathrm{P}}_{\mathrm{e}}}$ 

Since power and magnification have direct dependence, lens of greater power is required for magnification.

Hence, we use P₁ = 6D and P₃ = 12D for constructing a microscope. 

                       $\boxed{\mathrm{As} {\mathrm{P}}_0>{\mathrm{P}}_{\mathrm{e}}}$ 

Therefore, P₃ = 12D should serve as objective lens and P₁ = 6D should serve as eye-lens.