Let the height of the parallelogram be h and the height of COD be x
So the height of AOB will be h-x.
Opposite sides are equal
So bases if both the triangles is equal.
Ar(aob)+ar(cod)=1/2*(h-x)*base + 1/2 * x*base
Base* height= 40
Area of both triangles is 1/2*40
That is 20