The Old man was ________ weak to open the door .

A man and his wife is standing at the door.

The key for a door is in a bunch of 10 keys. A man attempts to open the door by trying keys at random discording the wrong key. The probability that the door is opened in the fifth trial is

The iron door of a building is 3m broad. It can be opened by applying a force of 100N normally at the middle of the door. Calculate (a) the torque needed to open the door, (b) the least force and its point of application to open the door.