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mspeople
Hello, 
The mass number is protons+neutrons=mass number. In this case, we have protons+nuetron=164.The atomic number is simply the number of protons so we have 43+neutrons=164. Subtracting 43 from both sides we get nuetrons=121. Hope this helps!
Incineroar
The atomic number is 43. This means it is Technetium. If its mass number is 164, then the number of neutrons is: 164 - 43 = 121. It has 121 neutrons.