Question

How many neutrons does element X have if its atomic number is 57 and its mass number is 168?

Answer 1

Mass number is # of protons + # of neutrons
since the atomic number is the number of protons, the number of protons is 57
so 168 = 57 + x
x = 111 neutrons

Answer 2

Mass number is number of protons + number of neutrons 

168 - 57 = Answer 

Answer = 111