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 60 cm from the optical centre?

According to Cartesian sign convention for a convex lens:Object distance ($u$) = $-60\text{cm}=-0.6\text{m}$ Image distance ($v$) = $+20\text{cm}=+0.2\text{m}$ (since it forms on the other side of the lens, it is a real image)Using the lens formula:
$$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$$
Substituting the values:
$$\frac{1}{f}=\frac{1}{0.2}-\frac{1}{-0.6}$$
$$\frac{1}{f}=5+\frac{1}{0.6}=5+1.67=6.67{\text{m}}^{-1}$$
Since Power ($P$) of a lens is given by $P=\frac{1}{f}$ (where $f$ is in meters):
$$P=+6.67\text{diopters (D)}$$