Answer
number of neutrons $=$ mass number $-$ number of protons $=239-94=145$
Work Step by Step
Strategy: The 239 in Pu- 239 is the mass number. The mass number $(A)$ is the total number of neutrons and protons present in the nucleus of an atom of an element. You can look up the atomic number (number of protons) on the periodic table.
Solution:
mass number $=$ number of protons $+$ number of neutrons
number of neutrons $=$ mass number $-$ number of protons $=239-94=145$
