Find the focal length and nature of lens which should be placed in contact with a lens of focal length 10 cm so that the power of the combination becomes 5D.

Power(P₂) of lens with 10 cm focal length = $\frac{100}{10}=10D$
Let Power of the lens is P₁
Total power of combination = P₁ +P₂
​5D=​ P₁ + 10D
P₁ = -5D
focal length = $$\begin{gathered} -\frac{1}{5}=-0.2\mathrm{m} \\ \Rightarrow -20 \mathrm{cm} \end{gathered}$$
Power of the lens is negative so the given lens is concave lens