A horizontal force of 25 N is necessary to just hold a block stationary against a wall. The coefficient of friction between the block and the wall is 0.4. The weight of the block is
                                       
 

Given horizontal force F=25  N and coefficient of friction between block and wall  $\left(\mu \right)=0.4$
We know that at equilibrium horizontal force provides the normal reaction to the block against the wall. Therefore, normal reaction to the block(R)=F=25N.
We also know that weight of the block (w)= Frictional force =  
$\mu R=0.4\times 25=10N.$