#### Answer

(a) 11
(b) 25
(c) 79
(d) 199

#### 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 boron is 5, and the number of neutrons in this particular atom is 6. So we add 5 to 6 to get a mass number of 11.
(b) The atomic number of magnesium is 12, and the number of neutrons in this particular atom is 13. So we add 12 to 13 to get a mass number of 25.
(c) The atomic number of bromine is 35, and the number of neutrons in this particular atom is 44. So we add 35 to 44 to get a mass number of 79.
(d) The atomic number of mercury is 80, and the number of neutrons in this particular atom is 119. So we add 80 to 119 to get a mass number of 199.