Answer: chlorine 17Cl35 has 18 neutrons, and 17Cl37 has 20 neutrons
Explanation: 17 is the atomic number of chlorine which tells us what is the number of protons in chlorine nucleus. 35 and 37 is the mass number of two isotopes of chlorine, it tells us what is the total number of protons and neutrons in atomic nucleus of two chlorine isotopes .
Now to know how many neutrons are in both isotopes we have to substract the number of protons from the mass number of each isotope
For 17Cl35 it will be 35- 17 what makes 18 and for 17 Cl 37 it will be 20 neutrons