Question 751969
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


{{{ 1-1/4-1/6=12/12-3/12-2/12=7/12}}}


Now if 21 represents 7/12 then 



{{{
(7/12)x=21
}}}


{{{(12/7)(7/12)x=(21/1)(12/7)}}}


And therefore there are 36 pieces of candy



:)