Gary puts orange, black & whites of candies in boxes for Halloween. After he fills a box 1/4 is orange, 1/6 is black and 21 are white. How many candies does he put in each box?

The fractions must add to 1 (the total of all the candies expressed as a fraction). So that the fraction of white candies must be 

 

Now if 21 represents 7/12 then 


 

 

And therefore there are 36 pieces of candy 


:)