If an isotope of an element has 33 neutrons and a mass number of 64, how many electrons must it have?

The mass number is the sum of the protons and neutrons

64 = #protons + #neutrons

Find the number of protons given the number of neutrons

64 = #protons + 33

#protons = 31

In a neutral element (such as this), the number of electrons equals the number of protons

#electrons = #protons

#electrons = 31