Let's start by declaring what we know.
15.0 mg = 0.0150 grams
Molecular formula of Prozac is C17H18F3NO
We can calculate the number of grams per mole by adding the molecular weights of the composing elements times the respective quantity of that element
Molar mass of carbon = 12.01 g/mol (17 carbon atoms)
Molar mass of hydrogen = 1.01 g/mol (18 hydrogen atoms)
Molar mass of fluroine = 19.00 g/mol (3 fluorine atoms)
Molar mass of nitrogen = 14.01 g/mol (1 nitrogen atom)
Molar mass of oxygen = 16.00 g/mol (1 oxygen atom)
The molar mass of prozac = 17(12.01 g/mol) + 18(1.01 g/mol) + 3(19.00 g/mol) + 1(14.01 g/mol) + 1(16.00 g/mol) = 309.36 g/mol
Now, we have the number of grams (0.0150 grams), and we have the number of grams per mole of a substance (309.36 grams/mole). To get the number of moles, we have to divide the mass of prozac we have, by the mass per mole of prozac.
(0.0150 grams)/(309.36 grams/mole) = 4.85 * 10^-5 moles of prozac