Question 336570
The probability before adding any blue balls is 10/(10+7+5)=10/22
Let x be the least number of blue marbles that must be added.
(10+x)/(22+x) > 1/2
10+x>(22+x)/2
20+2x>22+x
x>2 
Since x is greater than 2 then x must be 3.
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Ed