The answer would be:
-as a fraction:

, since there are a total of 31 kids and 15 boys.
-as a percentage: 48 percent. to find the percentage, you add both numbers (15+16) which is 31. As you can see, the fraction is

. now 15 divided by 31 is 0.48. 48%.
-as a ratio: 15:31 the explanation for this is the same as a fraction.