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# of neutrons = Mass Number - Number of Protons. To be able to apply this formula, refer to the atomic number and atomic weight of the elements. These two are listed in the Periodic Table of Elements. Take note that atomic number refers to the number of protons in an atom of an element.
The number of protons an atom has in its nucleus is given by the atomic number. A quick looks in the periodic table will show that aluminium has an atomic number equal to #13#. This means that any atom that is an isotope of aluminium will have #13# protons in its nucleus. Since you're dealing with a neutral atom, the number of electrons that surround the nucleus must be equal to the number of protons found in the nucleus. Therefore, the aluminium-27 isotope will have #13# electrons surrounding its nucleus. Finally, use the known mass number to determine how many neutrons you have.