Question 150851
Let's call the amount from beaker A, A.
Let's call the amount from beaker B, B.
The total final volume is 6 liters. 
1.{{{A+B=6}}}
The final concentration is 18%. 
2.{{{A(0.15)+B(0.20)=(A+B)(0.18)}}}
Let's multiply equation 2 by 100 to get rid of decimals.
2.{{{15A+20B=18(A+B)}}}
2.{{{15A+20B=18A+18B)}}}
2.{{{3A-2B=0}}}
We can use eq. 2 to solve for B in terms of A.
2.{{{3A-2B=0}}}
{{{2B=3A}}}
{{{B=(3/2)A}}}
Now substitute this into eq. 1 and solve for A.
1.{{{A+B=6}}}
{{{A+(3/2)A=6}}}
{{{(5/2)A=6}}}
{{{A=12/5}}}
Now back substitute to find B,
{{{B=(3/2)A}}}
{{{B=(3/2)(12/5)}}}
{{{B=18/5}}}
2.4 liters of solution A and 3.6 liters of solution B.