Hey Guideme,
If the object being acted on by the force is in equilibrium (not moving at all), that force is balanced by an equal one acting in the opposite direction to it. So that force of 20kgwt acting down the plane happens to be a component of the gravity force, and is the result of mg * sin40.
So we know that:
mg * sin40 = 20
Therefore:
Mass = 20/sin40
In this case, the mass equals 31.1145kg
*edit*
To find the pressure this weight exerts on the surface, you must find the normal force, or the component of the force acting perpendicular to the plane. As the object is in equilibrium, the pressure acting on the surface will be the same as the normal force. To find this we use the parallel force (20kgwt) and tan.
tan(theta) = opposite/adjacent
tan40 = 20/N
N = 20/tan40
N (pressure on surface) = 26.8416kgwt