A man balances himself in a horizontal position by pushing his hands and feet against two parallel walls. His centre of mass lies midway between the walls. The coefficients of friction at the walls are equal. Which of the following is not correct?

For the given figure, if block remains in equilibrium position then find frictional force between block and wall …….. $N$ 

A girl holds a book of mass $m$ against a vertical wall with a horizontal force $F$ using her finger, so that the book does not move. The frictional force on the book by the wall is

A block of mass $M$ is being pulled along rough horizontal surface. The coefficient of friction between the block and the surface is $\mu $. If another block of mass $M/2$ is placed on the block and it is again pulled on the surface, the coefficient of friction between the block and the surface will be