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There are $20$ balls of the same size in a box.  Lucas says: "$\frac12$ of them are red."  Peter says: "The probability of drawing a green ball without observing them is $\frac15$."  Claire says: "There are three colors of balls in the box: red, light blue, and green."  How many dark blue balls are there in the box? 
Overseas Competition->Knowledge Point->Calculation Modules->Fractions
There is no dark blue ball in the box. 