#### Answer

(a) 20
(b) 32
(c) 78
(d) 198

#### Work Step by Step

To calculate the mass number, you add the number of protons to the number of neutrons in an atom. In these problems, we are given the number of neutrons already. The number of protons is the same as the atomic number, which can be found by locating the element on the periodic table.
(a) The atomic number of fluorine is 9, and the number of neutrons in this particular atom is 11. So we add 9 to 11 to get a mass number of 20.
(b) The atomic number of sulfur is 16, and the number of neutrons in this particular atom is 16. So we add 16 to 16 to get a mass number of 32.
(c) The atomic number of arsenic is 33, and the number of neutrons in this particular atom is 45. So we add 33 to 45 to get a mass number of 78.
(d) The atomic number of platinum is 78, and the number of neutrons in this particular atom is 120. So we add 78 to 120 to get a mass number of 198.