A plane monochromatic light wave with intensity I0 falls normally on an opaque disc closing the first

Question:

A plane monochromatic light wave with intensity I0 falls normally on an opaque disc closing the first Fresnel zone for the observation point P. What did the intensity of light I at the point P become equal to after
(a) Half of the disc (along the diameter) was removed;
(b) Half of the external half of the first Fresnel zone was removed (along the diameter)?