Question

if the power of A lens is 4dioptres find its focal length

Answer 1

power=1/focal length (in meters)
let the focal length be x 
4=1/x
x=1/4
x=0.25 m
x=25 cm
hence the focal length of the lens is 25 cm

Answer 2

p=4 dioptre p=1/f so 1/f=4 so f =0.25