In probability, you gather data through statistics and from there, determine the chances of how likely an event is going to happen out of the total number of data you've collected. Hence, probability is always expressed as part of a whole. This can be in terms of fraction, decimal or in percentage.
You are given statistics: 12 girls and 16 boys. You solve for the total which is 12+16 = 28. This would be the denominator of your probability. For the numerator, we use the statistics for the girls. Therefore, the probability of selecting a girl to dive first is
Probability = 12/28 = 3/7 or 42.86%