Z=15 means that element is phosphorus . and it contains 15 electrons and 15 protons. and its atomic weight is 31.

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Z = 15 means there are 15 protons in the element
As number of proton is equal to number of electrons, there are 15 electrons.
From z = 15, you also know that it is Phosphorus. 
But z= 15 does not provide information about its atomic mass(mass number). You have to know the number of neutrons init. Number of neutrons is 16 and hence the mass number is (15+16) = 31.