For a normal eye, the cornea of eye provides a converging power of 40 D and the least converging power of the eye lens behind the cornea is P diopter. Using this information, the distance between the retina and the cornea-eye lens can be estimated as $\frac{5}{3}$cm. Then value of P in diopter is 

Focal length of the combined lens is the distance between retina and cornea eye lens =5/3 cm 

Converging power of cornea$P_c=+40D$

Least converging power of eye lens,$P_e=+P D$

Power of the eye-lens,$P=P_c+P_e=40D+P D=(40+P)D$

Power of the eye lens$P=\frac{1}{Focallengthoftheeyelens\left(f\right)}$

$f=\frac{1}{P}=\frac{1}{\left(40+P\right)D}==\frac{5}{300}m$

solving we get P=20 D