Question

How many neutrons does an element have if its atomic number is 45 and its mass number is 161

Answer 1

Proton and electron same
proton and the atomic number is same
So mass (-) atomic number
which is 161-45= 116
answer is 116 :)

Answer 2

Here is the formula that you need to know for this problem:
Number of proton (atomic number) + Number of neutrons = Mass number

So,

45  +  Number of Neutrons = 161

Number of neutrons = 161 - 45

Number of Neutrons = 116