Boxers and collies added together gives you 12:16. Divide them both by 4 and you get 3:4.
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The number of student in class 4 (6) divided by the total number of kids in the entire class (18) gives you 0.33333333333. Multiply that by 100 and it gives you 33.3333333333. As a percentage it is 33 1/2 %
