Question

number of neutrons in zinc ion with mass number 70 is

Answer 1

40 neutrons and 30 protons make up zinc atom having mass number 70

Answer 2

mass no = 70 
→Number of protons=30
→No of neutrons + No of protons = 70
→As we know that no.of protons does not change
→Neutrons+Protons=70
→Neutrons+30=70
⇒Neutrons=40