First, take into account that the value of the joint relative frequency, for residents of Irondale who prefer gelato are 0.16.
Next, consider that the total joint relative frequency of resident in Irondale is:
0.12 + 0.16 = 0.28
Next, calculate the quotient in between the relative frequency 0.16 and the total relative frequency 0.28:
0.16/0.28 = 0.57
Then, the probability is 0.57x100 = 57%