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There are $12$ black balls, $27$ red balls, and $11$ blue balls in an opaque box. The balls are the same except for their colors. Bob adds several red balls in the box and mixes the balls. Now, if the probability of taking out a black ball is $\frac15$, how many red balls does Bob add? 
Overseas Competition->Knowledge Point->Counting Modules->Statistics and Probability
$\frac15=\frac{12}{60}$, which means there are $60$ balls in total. Thus, Bob adds $60-12-27-11=10$ red balls. 