​f1​=v1​1​+u1​−151​=v1​1​−401​⇒v1​1​=−151​+401​v1​=−24 cm​ When object is displaced by 20 cm towards mirror.Now,​u2​=−20f1​=v2​1​+u2​1​−151​=v2​1​−201​v2​1​=201​−151​v2​=−60 cm​ So, image shifts away from mirror by =60−24=36 cm.