Camera Calibration and the Pinhole Model

A robot uses a calibrated pinhole camera with focal length $f = 800$ pixels (i.e., $f_x = f_y = 800$). A planar object lies on a table directly in front of and parallel to the image plane, at a depth of $Z = 4.0$ meters from the camera's optical center. The object has a true physical width of $0.50$ meters. Assuming no lens distortion and that the object is centered, how wide (in pixels) will the object appear in the captured image?