Hello!
There are two parts of a fraction. The top part is called the 'numerator' and the bottom part is the 'denominator'.
The numerator is how many of something there is, and the denominator is the total amount of something, or, 1 whole.
In this case, 5 would be the numerator because there are 5 YELLOW marbles, and 12 would be the denominator because that is the total amount of marbles.
So, your answer is:
5/12 of the marbles are yellow.
