Answer:
The chlorine-37 has 20 neutrons.
Explanations:
1) When an atom of chlorine-35, then it contains 18 neutrons.
 mass = 17 protons + 18 neutrons = 35
2) When an atom of chlorine-37 then it contains 20 neutrons
 mass = 17 protons + 20 neutrons = 37 
Note: Adding or removing a neutron from an atom's nucleus creates isotopes of a particular element