the mass number can be calculated using the following rule:
mass number = number of protons + number of neutrons
we have the number of neutrons given as 157 neutrons
now for the protons, we know that in the neutral state, number of protons is equal to the number of electrons. Since this ion has a +1 charge, this means that the ion has one proton more than the number of electrons.
Therefore, number of protons = 109 + 1 = 110 protons
Substituting in the above equation, we can calculate the mass number:
mass number = 110 + 157 = 267
