12 is the denominator because it represents the total number of times Nicanor selects a diving ring from the bag
Explaination:
Nicanor randomly selects a ring from the bag a total of 12 times. 3 times he selects a red ring, 4 times a yellow one, and 5 times a blue one.
When he selects a red ring, he selects it 3 times out of the 12 times total, which is represented as the fraction 3/12.
When he selects a yellow ring, he selects it 4 times out of the 12 times total, which is represented as the fraction 4/12.
When he selects a blue ring, he selects it 5 times out of the 12 times total, which is represented as the fraction 5/12.
Hope this helps you!