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Answer: Holmium-165 = Er-165 + electron
Explanation:
Beta decay is the decay of a neutron into a proton and electron. This will increase the number of protons in the Holmium nucleus by one, making it Er, Erbium, with the same atomic mass.
Holmium-165 = Er-165 + electron
The mass is essentially the same, as the proton is assigned the same mass as the neutron it came from.