Find the power of a convex lens if the image formed is at a distance of 20 cm from the lens when the object is placed on the other side of the lens at 25 cm from the optical centre?

According to the Cartesian sign convention for a convex lens:Object distance ($u$) = $-25\text{cm}=-0.25\text{m}$ Image distance ($v$) = $+20\text{cm}=+0.20\text{m}$ (since the image is formed on the other side of the lens)Using the lens formula:
$$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$$
The power of a lens ($P$) in diopters is given by $P=\frac{1}{f}$ (where $f$ is in meters):
$$P=\frac{1}{v}-\frac{1}{u}$$
$$P=\frac{1}{0.20}-\frac{1}{-0.25}$$
$$P=5-(-4)=5+4=9\text{diopters}$$
Therefore, the power of the convex lens is $9\text{diopters}$.