Answer
129
Explanation
Mass number is the total or sum of number of protons and neutrons (together known as nucleons) in an atomic nucleus.
The atomic number is the number of protons in an atom.
Mass number = number of protons + number of neutrons
Number of protons = atomic number = 61
Number of neutrons = 68
Therefore, the mass number of the element = 61 + 68 = 129 u
