Intensity of radiation at a distance of 4 cm is R. At the distance of 2 cm, the intensity will be?

The intensity of an X-ray beam (the number of photons per cross sectional area per unit of exposure time) depends on the distance of the measuring device from the focal spot. For a given beam, the intensity is inversely proportional to the square of the distance from the source. White and Pharoah's Oral Radiology Principles and Interpretation 8th edition