Question

Element A has 15 protons. Determine the number of neutrons in its isotope with mass number 33.

Answer 1

A=Z+N
A=mass number  (protons + neutrons)
Z=atomic number (number of protons).
N=number of neutrons.

Data:
Z=15
A=33

Then:

33=15+N
N=33-15=18

Answer: the number of neutrons in this element A will be 18.

Answer 2

Number of neutrons is mass number minus number of protons. 
So the number of neutrons is 18.