An Astronomical telescope having an objective of focal length 100cm  is focused on the moon. The distance through which the eye piece should be pulled back to focus an object situated at a distance 80 m from the objective is

as moon is very far distance object image will be at the focus I.e, it is at a distance 1 m from the objectivewhen object is 80 m from the objective image will be180+1v=11⇒⇒1v=1-180=7980 v=8079  so image shifted by 1-8079=179 away from the objective so eyepiece also has to be pulled away by 179m or 10079cm