Pressure is defined as force exerted per unit area (perpendicular to the direction of application of force, or the area over which the force actually acts). Here the force is exerted by a boy of mass 45 kg. Given the mass of boy, we can use Newton's Second Law of motion to determine the force as the product of mass and acceleration (g or acceleration due to gravity, in this case) as:
Force exerted by the boy = mg = 45 kg x 9.81 m/s^2 = 441.45 N
Area over which the force acts = side^2 (for a square)
= (0.5 m)^2 = 0.25 m^2.
Thus, the pressure exerted by the boy = force/area = 441.45 N/ 0.25 m^2 = 1765.8 N/m^2.
Thus, the boy exerts a pressure of 1765.8 N/m^2 on the square wooden block.
Hope this helps.