Question

how many neutrons does element x have if its atomic number is 22 and its mass number is 84

Answer 1

Mass Number (given as 84) = # of protons + # of neutrons

Atomic Number (given as 22) = # of protons

Plug in values to first equation:
84 = 22 + n (where n equals to # of neutrons)

Solve for n (subtract 22 from both sides)
84 - 22 = n

Answer: n = 62