(i) When we change u, by moving the object, distance of image v changes. But focal length of mirror, i.e., f is not affected.
(ii) Here, `R=40cm`, `f=R/2 =40/2 = 20`cm.
(iii) In day to day life, an object is like a situation that comes up and image is like the response we give to it. We are like a mirror. Our nature/curvature determines our focal length. Response to a particular situation will depend upon our nature. One the contray, our nature/focal length is not affected by the situation that comes up.