The number of protons in a given atom are the same as its atomic number. In fact, atomic number is the number of protons in the nucleus of an atom. For example, sodium has an atomic number of 11 and has got 11 protons in its nucleus.
In the given case, the atomic number of the given atom is 15, hence its nucleus contains 15 protons.
The number of protons and electrons are the same in atom and that is what keeps it neutral. In the second case, the atomic number is 20. Hence, the atom will contain a total of 20 protons in its nucleus. To counter this charge, the atom will also contain 20 electrons. Note that there are no electrons inside the nucleus, electrons only orbit the nucleus.
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