Can someone help explain this to me? Im really confused because I thought that protons and neutrons are equal in a neutral atom, how are you supposed to find the number of neutrons? How is my answer wrong? The only element close to having a mass number of 15 is Nitrogen so I don't see how the answer would be Oxygen, since oxygen has a mass number of 16.

Pegmatites are igneous rocks in which the individual minerals are very large. typically, the minerals are all light-colored quartz, feldspar and muscovite. if you were given a black and white photograph of a pegmatite in a quarry (where the rock has been blasted and broken), what physical properties could you use to identify those three minerals in this hypothetical photo? describe each mineral and the specific diagnostic properties. be specific.