Question 406731
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Hi
Let x represent represent the amount of 10% solution
Question states*** 
Mixing together to form (15L+x) 20% solution
 .10x + .40*15L = .20(15L+x)
Solving for x 
    .20*15L = .10x
     .20*15L/.10 = x = 30L, the amount of the 10% solution

CHECKING our Answer***
   3 + 6 = .20*45 = 9